cbse class 10 heredity  and evolution notes

           Class 10 Heredity and Evolution – Gist                         
                         of lesson

Genetics : Branch of science that deals with Heredity and variation.

Heredity : It means the transmission of features / characters/ traits from one generation to the next generation.

Variation : The differences among the individuals of a species/population are called variations.

Mendel and His Work on Inheritance

Gregor Johann Mendel started his experiments on plant breeding and hybridization. He proposed thelaws of inheritance in living organisms. Mendel was known as Father of Genetics

Plant selected by Mendel : Pisum sativum (garden pea). Mendel used a number of contrasting characters for garden pea.

Following are the seven pairs of contrasting characters in Garden Pea

(TABLE OF CONTRASTING CHARACTERS. SEVEN PARTS)
CHARACTER	DOMINANT TRAIT	RECESSIVE TRAIT
Flower colour	Violet	White
Flower position	Axial	Terminal
Seed colour	Yellow	Green
Seed shape	Round	Wrinkled
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Height of plant	Tall	Dwarf/Short

Mendels Experiments : Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time)

Monohybrid Cross : Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross. Example : Cross between a tall and a draft plant (short).

Observations of Monohybrid Cross

1. All F1 progeny were tall (no medium height plant (half way characteristic)

2. F2 progeny ¼ were short, ¾ were tall

3. Phenotypic ratio F2 – 3:1 (3 tall : 1 short)

Genotypic ratio F2 – 1 : 2 :1  = TT : Tt : tt

Conclusions

1. TT and Tt both are tall plants while tt is a short plant.

2. A single copy of T is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short.

3. Characters/Traits like 'T' are called dominant trait (because it express itself) and ‘t’ are recessive trait (because it remains suppressed)

Dihybrid Cross : A cross macle between two plants having two pairs of contrasting characters is called dihybrid cross.

PARENT GENERATION  ---> ROUND GREEN SEEDS x WRINKLED YELLOW SEEDS      

Phenotypic Ratio

Observations

1. When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.

2. Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow & wrinkled green) seeds plants in the ratio of 9:3:3:1

Conclusions

1. Round and yellow seeds are DOMINANT characters

2. Occurrence of new phenotypic combinations show that genes for round and yellow seeds areinherited independently of each other

SEX DETERMINATION : Phenomenon of decision or determination of sex of an offspring

FACTORS Responsible for Sex Determination

1. Environmental : In some animals the temperature at which the fertilised eggs are kept decides the gender. eg. in Turtle                                                                                  2. Genetic : In some animals like humans gender or individual is determined by a pair of chromosome called sex chromosome  XX – Female and XY – Male

This shows that half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless whether they are boys or girls. Thus sex of children will be determined by what they inherit from their father, and not from their mother.

EVOLUTION

Evaluation is the sequence of gradual changes which takes place in the primitive organisms, over millions of years, in which new species are produced.

Situation-I

		Group of red beetles
	Colour variation arises during reproduction
All beetles red except one that is green			One beetle Green Reproduction
Crows feed on red beetle			Progeny beetles green
No. of beetles reduces			Crow could not feed on green beetles as they got camouflaged in green bushes
			Number of green beetles increases

Situation 1 : Green beetles got the survival advantage or they were naturally selected as they were not visible in green bushes. This natural selection is exerted by crows resulting in adaptations in the beetlesto fit better in their environment  

Situation 2 : Blue beetles did not get survivals advantage. Elephant suddenly caused major havoc in beetle population otherwise their number would have been considerably large.

From this we can conclude that accidents can change the frequency of some genes even if they do not get survival advantage: This is called genetic drift and it leads to variation.

Characters or traits of an organism are controlled by the genes

A Section of DNA (cellular)

Gene

Provides information

For synthesis of Proteins

Proteins controls a character

Example :

Gene T		responsible for synthesis of efficient enzyme (Protein)		More production of growth hormone		Results in Tall Plants
Gene t		responsible for synthesis of less  efficient enzyme		Less production of growth hormone		Results in short Plants

Genetic drift. It leads to diversity without any adaptation

SITUATION-III

Group of red beetles

Habitat of beetles (bushes)

Suffer from plant disease

Average weight of beetles

decreases due to poor nourishment

Number of beetles kept on reducing

Later plant disease gets eliminated

Number and average weight of the beetles increases again

Situation 3 : No genetic change has occurred in the population of beetle. The population gets affected for a short duration only due to environmental changes.

ACQUIRED AND INHERITED TRAITS

Acquired Traits	Inherited Traits
1. These are the traits which are developed in an individual due to special conditions	1. These are the traits which are passed from one generation to the next.
2. They cannot be transferred to the progeny	2. They get transferred to the progeny.
3. They cannot direct evolution eg. Low weight of starving beetles.	3. They are helpful in evolution. eg. Colour of eyes and hair

SPECIATION

Micro evolution : It is the evolution which is on a small scale. eg. change in body colour of beetles.

The process by which new species develop from the existing species is known as speciation.

Speciation : it is the process of formation of new species. 

Species : A group of similar individuals within a population that can interbreed and produce fertile offspring.

Geneflow : It is exchange of genetic material by interbreeding between populations of same species or individuals

WAYS BY WHICH SPECIATION TAKES PLACE

Speciation takes place when variation is combined with geographical isolation.

Gene flow : occurs between population that are partly but not completely separated

GENETIC DRIFT

It is the random change in the frequency of alleles (gene pair) in a population over successive generations.

*Natural Selection : The process by which nature selects and consolidate those organisms which are more suitably adapted and possesses favorable variations 

Genetic drift takes place due to 

(a) Severe changes in the DNA  (b) Change in number of chromosomes

Evolution and classification

Both evolution and classification are interlinked.

1. Classification of species is reflection of their evolutionary relationship.

2. The more characteristic two species have in common the more closely they are related.

3. The more closely they are related, the more recently they have a common ancestor.

4. Similarities among organisms allow us to group them together and to study their characteristic.

TRACING EVOLUTIONARY RELATIONSHIPS

(Evidences of Evolution)

I. Homologous Organs : (Morphological and anatomical evidences. These are the organs that have same basic structural plan and origin but different functions.

Homologous organs provides evidence for evolution by telling us that they are derived from the sameancestor.

Example :

Forelimb of Horse	(Running)	Same basic structural but different functions perform.
Winds of bat	(flying) plan,
Paw of a cat	(walk/scratch/attack)

II. Analogous Organs : These are the organs that have different origin and structural plan but same function example :
Example : Analogous organs provide mechanism for evolution.

Wings of bat       elongated fingers with skin folds

Wings of bird     Feathery covering along the arm

Different basic structure, but perform similar function i.e., flight

III. Fossils : (Palaeontological evidences) : The remains and relics of dead organisms of the past.

FOSSILS ARE PRESERVED TRACES OF LIVING ORGANISMS

Fossil Archaeopteryx possess features of reptiles as well as birds. This suggests that birds have evolved from reptiles. Examples of Fossils

AMMONITE - Fossil-invertebrate                
TRILOBITE - Fossil-invertebrate

KNIGHTIA - Fossil-fish                                 
RAJASAURUS - Fossil dinosaur skull

AGE OF THE FOSSILS

I. Deeper the fossil, older it is.                                                                                                           
II. Detecting the ratios of difference of the same element in the fossil material i.e. Radio-carbon dating[C-(14) dating)

Evolution by stages : Evolution takes place in stages ie bit by bit over generations.

I. Fitness advantage

Evolution of Eyes

Evolution of complex organs is not sudden it occurs due to minor changes in DNA, however takes place bit by bit over generations. 

Flat worm has rudimentary eyes

Insects have compound eyes                 enough to give fitness advantage

Humans have binocular eyes

II. Functional Advantage

Evolutions of feathers :

Feathers provide insulation in cold weather but later they might become useful for flight.

Example : Dinosaurs had feathers, but could not fly using feathers. Birds seem to have later adapted the feathers to flight.

Evolution by Artificial Selection :

Humans have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artificial selection. eg

(i) From wild cabbage many varieties like broccoli, cauliflower, red cabbage, kale, cabbage and kohlrabi were obtained by artificial selection.

(ii) Wheat (many varieties obtained due to artificial selection).

Molecular Phylogeny

Þ  It is based on the idea that changes in DNA during reproduction are the basic events in evolution

Þ Organisms which are more distantly related will accumulate greater differences in their DNA

                                         HUMAN EVOLUTION

Tools to Study Human Evolutionary Relationship

Excavating         Time dating        Fossils    Determining DNA Sequences

Although there is great diversity of human forms all over the world get all humans are a single species.

MENDEL'S GENETIC LAWS

Once upon a time (1860's), in an Austrian monastery, there lived a monk named Mendel, Gregor Mendel. Monks had a lot of time on there hands and Mendel spent his time crossing pea plants. As he did this over & over & over & over & over again, he noticed some patterns to the inheritance of traits from one set of pea plants to the next. By carefully analyzing his pea plant numbers (he was really good at mathematics), he discovered three laws of inheritance.

Mendel's Laws are as follows:

1. the Law of Dominance
2. the Law of Segregation
3. the Law of Independent Assortment

Now, notice in that very brief description of his work that the words "chromosomes" or "genes" are nowhere to be found.  That is because the role of these things in relation to inheritance & heredity had not been discovered yet. What makes Mendel's contributions so impressive is that he described the basic patterns of inheritance before the mechanism for inheritance (namely genes) was even discovered.

There are a few important vocabulary terms we should iron-out before diving into Mendel's Laws.

GENOTYPE = the genes present in the DNA of an organism.  We will use a pair of letters (ex: Tt or YY or ss, etc.) to represent genotypes for one particular trait.  There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from mama organism & the other comes from papa organism, so every offspring gets two codes (two letters).

Now, turns out there are three possible GENOTYPES - two big letters (like "TT"), one of each ("Tt"), or two lowercase letters ("tt"). Since WE LOVE VOCABULARY, each possible combo has a term for it.When we have two capital or two lowercase letters in the GENOTYPE (ex: TT or tt) it's called HOMOZYGOUS ("homo" means "the same").  Sometimes the term "PURE" is used instead of homozygous.
When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means "other").  Just to confuse you, a heterozygous genotype can also be referred to as HYBRID. OK?
Let's Summarize:



Genotype = genes present in an organism (usually abbreviated as two letters)
TT = homozygous = pure	Tt = heterozygous = hybrid	tt = homozygous = pure

PHENOTYPE = how the trait physically shows-up in the organism.  Wanna know the simplest way to determine an organism's phenotype ?  Look at it.  Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers.

 

ALLELES = (WARNING - THIS WORD CONFUSES PEOPLE; READ SLOW) alternative forms of the same gene.  Alleles for a trait are located at corresponding positions on homologous chromosomes.

Remember just a second ago when explaining genotypes I said that "one code (letter) comes from ma & one code (letter) comes from pa"? Well "allele" is a fancy word for what I called codes.

For example, there is a gene for hair texture (whether hair is curly or straight).  One form of the hair texture gene codes for curly hair.  A different code for of the same genemakes hair straight.  So the gene for hair texture exists as two alleles --- one curly code, and one straight code.Let's try & illustrate with a diagram.
In this picture the two "hot dog" shapes represent a pair of homologous chromosomes.  Homologous chromosomes are the same size & have the same genetic info (genes).  Each letter in the diagram stands for an allele (form of a gene).  What's important to notice is that the letters can be in different forms (capital or lowercase) --- that is what we mean by allele --- and that the letters are lined-up in the same order along each hot dog --- I mean homologous chromosome. The "a-forms" are in corresponding positions, so are the "B-forms", the "c" alleles, the "d" alleles, etc. etc. OK?
Reread that "allele" definition again & study the picture.
Getting back to our abbreviations, we could use a "C" for the curly allele, and a "c" for the straight allele.  A person's genotype with respect to hair texture has three possiblilties: CC, Cc, or cc.  So to review some vocab, homozygous means having two of the same allele in the genotype (2 big or 2 little letters --- CC or cc).  Heterozygous means one of each allele in the genotype (ex: Cc).
Now I could tell you which genotypes create curls & which do not, but then I'd be stealing some of Mr. Mendel's thunder.  More on that in a minute ........

Vocabulary Review Questions

1. Which of the following is a possible abbreviation for a genotype?

A. BC
B. Pp
C. Ty
D. fg

2. What is the best way to determine the phenotype of the feathers on a bird?

A. analyze the bird's DNA (genes)
B. look at the bird's feathers
C. look at the bird's beak
d. examine the bird's droppings

3. Which of the following pairs is not correct?

A. kk = hybrid
B. hybrid = heterozygous
C. heterozygous = Hh
D. homozygous = RR

4. The genes present in an organism represent the organism's __________.

A. genotype
B. phenotype
C. physical traits

5. Which choice represents a possible pair of alleles?

A. k & t
B. K & T
C. K & k
D. K & t

6. How many alleles for one trait are normally found in the genotype of an organism?

A. 1
B. 2
C. 3
D. 4

7. Which statement is not true?

A. genotype determines phenotype
B. phenotype determines genotype
C. a phenotype is the physical appearance of a trait in an organism
D. alleles are different forms of the same gene

<answers are here>

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 Ma-Ma-Ma-Ma-Mendel's First Law

The Law of Dominance
Stated "simply" it goes like so:
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting.  When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall.  Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants.  The same was true for other pea traits:

Parent Pea Plants	F1 Pea Plants
tall stem x short stem	all tall stems
yellow seeds x green seeds	all yellow seeds
green pea pods x yellow pea pods	all green pea pods
round seeds x wrinkled seeds	all round seeds
axial flowers x terminal flowers	all axial flowers

So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait.  So, he said to himself,

"Greg, there is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore Greg ol' boy, when the factors are mixed, the tall factor seems to DOMINATE the short factor".

Now, in our modern wisdom, we use "allele" or "gene" instead of what Mendel called "factors".   There is a gene in the DNA of pea plants that controls plant height (makes them either tall or short).  One form of the gene (allele) codes for tall, and the other allele for plant height codes for short.  For abbreviations, we use the capital "T" for the dominant tall allele, and the lowercase "t" for the recessive short allele.Let's revisit the three possible genotypes for pea plant height & add some MORE VOCABULARY.

Genotype Symbol	Genotype Vocab	Phenotype
TT	homozygous DOMINANT or pure tall	tall
Tt	heterozygous or hybrid	tall
tt	homozygous RECESSIVE  or pure short	short

Note: the only way the recessive trait shows-up in the phenotype is if the geneotype has 2 lowercase letters (i.e. is homozygous recessive).
Also note: hybrids always show the dominant trait in their phenotype (that, by the way,  is Mendel's Law of Dominance in a nutshell).

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The PUNNETT SQUARE (P-Square for short)

OK, now is as good of time as any to introduce you to a new friend, the Punnett Square.  This little thing helps us illustrate the crosses Mendel did, and will assist you in figuring out a multitude of genetics problems.

We will start by using a P-Square to illustrate Mendels Law of Dominance.  Recall that he "discovered" this law by crossing a pure tall pea plant & a pure short pea plant.  In symbols, that cross looks like this:

Parents (P):  TT x tt
where T = the dominant allele for tall stems
 &  t = recessive allele for short stems

The P-Square for such a cross looks like this:
Inside the 4 boxes are the possible genotypes (with respect to plant height) of the offspring from these parent pea plants.  In this case, the only possible genotype is Tt (heterozygous).  In hybrids, the dominant trait (whatever the capital letter stands for) is the one that appears in the phenotype, so all the offspring from this cross will have tall stems.

To "fill in the boxes" of the Punnett Square, say to yourself "letter from the left & letter from the top".  The "t" from the left is partnered with the "T" from the top to complete each of the four squares.

A summary of this cross would be:

Parent Pea Plants  (P Generation)		Offspring  (F1 Generation)
Genotypes: TT x tt	Phenotypes: tall x short	Genotypes: 100% Tt	Phenotypes: 100% tall

Now, a helpful thing to recognize is this: ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT, AND ALL THEIR OFFSPRING RESEMBLE ONLY ONE OF THE PARENTS, YOU ARE DEALING WITH MEDEL'S LAW OF DOMINANCE. All the offspring are heterozygous for the trait, one parent is homozygous dominant, and the other is homozygous recessive.

Does setting up & using the Punnett Square confuse you? Would you like to see a step-by-step "how to" about the good ol' p-square?  If you said "yes", then check this out:  "The Punnet Square (in baby steps)".For some practice Punnett Square problems visit my very own: "P-Square Practice Page". Don't forget to come back & learn more about Mendel!

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 Ma-Ma-Ma-Ma-Mendel's Second Law

The Law of Segregation
Goes like so: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result.  Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome.  So, at one point he takes theoffspring of a previous cross & crosses them.  Ooooooooh ............
Recall that his original cross for the tall & short pea plants was:

	Parents	F1 Offspring
Genotype(s)	TT x tt	100% Tt
Phenotype(s)	tall x short	100% tall

So, he takes two of the "F1" generation (which are tall) & crosses them.  I would think that he is figuring that he's gonna get all tall again (since tall is dominant).  But no!  Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself,

"Greg ol' boy, the parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall. To get short plants from these parents, the tall & short factors must separate, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization."

I think it's easier to picture this law by using a p-square.  Our cross is two hybrid parents, Tt x Tt.
The punnet square would look like this:
Now, when completing a Punnet Square, we model this "Law of Segregation" every time.  When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called "MEIOSIS".  Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm. Sometimes the term "GAMETOGENESIS" is used instead of meiosis.  Scientists love vocabulary (sorry).You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example).
Our summary:

Parent Pea Plants  (Two Members of F1 Generation)		Offspring  (F2 Generation)
Genotypes:Tt x Tt	Phenotypes:tall x tall	Genotypes: 25% TT 50% Tt 25% tt	Phenotypes:75% tall 25% short

A helpful thing to recognize: Any time two parents have the same phenotype for a trait  but some of their offspring look different with respect to that trait,  the parents must be hybrid for that trait.

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 Ma-Ma-Ma-Ma-Mendel's Third Law

The Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.OK. So far we've been dealing with one trait at a time.  For example,  height (tall or short), seed shape (round or wrinkled), pod color (green or yellow), etc.  Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another.  In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.
Please note my emphasis on the word "different".  Nine times out of ten, in a question involving two different traits, your answer will be "independent assortment".  There is a big ugly punnet square that illustrates this law so I guess we should take a look at it.  It involves what's known as a "dihybrid cross", meaning that the parents are hybrid for twodifferent traits.
The genotypes of our parent pea plants will be:
RrGg x RrGg where
"R" = dominant allele for round seeds
"r" = recessive allele for wrinkled seeds
"G" = dominant allele for green pods
"g" = recessive allele for yellow podsNotice that we are dealing with two different traits: (1) seed texture (round or wrinkled) & (2) pod color (green or yellow).  Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).
We need to "split" the genotype letters & come up with the possible gametes for each parent.  Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter. So each gamete should have one "are" and one "gee" for a total of two letters.  There are four possible letter combinations: RG, Rg, rG, and rg. These gametes are going "outside" the p-square, above 4 columns & in front of 4 rows.  We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees").

This is what it looks like: 

	RG	Rg	rG	rg
RG	RRGG round	RRGg round	RrGG round	RrGg round
Rg	RRGg round	RRgg round	RrGg round	Rrgg round
rG	RrGG round	RrGg round	rrGG wrinkled	rrGr wrinkled
rg	RrGg round	Rrgg round	rrGg wrinkled	rrgg wrinkled

The results from a dihybrid cross are always the same:
9/16 boxes (offspring) show dominant phenotype for both traits (round & green),
3/16 show dominant phenotype for first trait & recessive for second (round & yellow),
3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green), &
1/16 show recessive form of both traits (wrinled & yellow).

So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod.  The different traits do not influence the inheritance of each other.  They are inherited INDEPENDENTLY.

Interesting to note is that if you consider one trait at a time, we get "the usual" 3:1 ratio of a single hybrid cross (like we did for the LAw of Segregation). For example, just compare the color trait in the offspring; 12 green & 4 yellow (3:1 dominant:recessive).  Same deal with the seed texture; 12 round & 4 wrinkled (3:1 ratio).  The traits are inherited INDEPENDENTLY of eachother --- Mendel's 3rd Law.

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Summary: I would like to summarize Mendel's Laws by listing the cross that illustrates each. LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods There you have them, Mendel's huge contributions to the world of science.  A very smart cookie.  His work has stood the test of time, even as the discovery & understanding of chromosomes & genes has developed in the 140 years after he published his findings.  New discoveries have found "exceptions" to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong.

Review Questions

1. Which cross would best illustrate Mendel's Law of Segregation?

A. TT x tt
B. Hh x hh
C. Bb x Bb
D. rr x rr

2. In the cross Yy x Yy, what percent  of offspring would have the same phenotype as the parents?

A. 25%
B. 50%
C. 75%
D. 100%

3. In a certain plant, purple flowers are dominant to red flowers.  If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants?

A. PP x Pp
B. Pp x Pp
C. pp x PP
D. pp x pp

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A white-flowered plant is crossed with a pink-flowered plant.  All of the F1 offspring from the cross are white.

4. Which phenotype is dominant? 
5. What are the genotypes of the original parent plants? 
6. What is the genotype of all the F1 offspring? 
7. What would be the percentages of genotypes & phenotypes if one of the white F1 plants is crossed with a pink-flowered plant? 
8. Which of Mendel's Laws is/are illustrated in this question? 

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9. Crossing two dihybrid organisms results in which phenotypic ratio?

A. 1:2:1
B. 9:3:3:1
C. 3:1
D. 1:1

10. The outward appearance (gene expression) of a trait in an organism is referred to as:

A. genotype
B. phenotype
C. an allele
D. independent assortment

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11. In the homologous chromosomes shown in the diagram, which is a possible alleleic pair?

A. cD
B. Ee
C. AB
D. ee

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12. The phenotype of a pea plant can best be determined by:

A. analyzing its genes
B. looking at it
C. crossing it with a recessive plant
D. eating it

13. Mendel formulated his Law of Segregation after he had:

A. studied F1 offspring
B. studied F2 offspring
C. produced mutations
D. produced hybrids

14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance?

A. TT x tt
B. TT x Tt
C. Tt x Tt
D. tt x tt

15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair.  This mating illustrates which of Mendel's Laws?

A. Dominance
B. Segregation
C. Independent Assortment
D. Sex-Linkage

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Answers Area 

Vocabulary Term Review Questions - CORRECT ANSWERS ARE UNDERLINED

1. Which of the following is a possible abbreviation for a genotype?

A. BC
B. Pp - genotypes are made up of 2 of the same letter (either 2 capital, 2 lowercase, or one of each)
C. Ty
D. fg

2. What is the best way to determine the phenotype of the feathers on a bird?

A. analyze the bird's DNA (genes)
B. look at the bird's feathers - "phenotype of the feathers" means what the feathers look like, so look at 'em
C. look at the bird's beak
d. examine the bird's droppings

3. Which of the following pairs is not correct?

A. kk = hybrid - Kk would be hybrid (one capital, one lowercase of the same letter)
B. hybrid = heterozygous
C. heterozygous = Hh
D. homozygous = RR

4. The genes present in an organism represent the organism's __________.

A. genotype
B. phenotype
C. physical traits

5. Which choice represents a possible pair of alleles?

A. k & t
B. K & T
C. K & k - allele means 2 forms of the same gene. so this choice shows 2 forms of the same letter K or k
D. K & t

6. How many alleles for one trait are normally found in the genotype of an organism?

A. 1
B. 2 - one allele is inherited from each parent
C. 3
D. 4

7. Which statement is not true?

A. genotype determines phenotype - (note that the environment does play a role in influencing phenotype too)
B. phenotype determines genotype
C. a phenotype is the physical appearance of a trait in an organism
D. alleles are different forms of the same gene - (see question #5)

Review Questions - ANSWERED & EXPLAINED

1. Which cross would best illustrate Mendel's Law of Segregation?

A. TT x tt
B. Hh x hh
C. Bb x Bb - both parent show dominant trait, but some recessive offspring will be produced (each parent carries a "b")
D. rr x rr

2. In the cross Yy x Yy, what percent  of offspring would have the same phenotype as the parents?

A. 25%
B. 50%
C. 75% - in the completed p-square, 3 of 4 boxes will have at least 1 "Y", producing the dominant phenotype (same as parents)
D. 100%

3. In a certain plant, purple flowers are dominant to red flowers.  If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants?

A. PP x Pp
B. Pp x Pp - for any offspring to be recessive, each parent MUST have at leat one "p"
C. pp x PP - only one parent is purple, this CAN'T be an answer
D. pp x pp - neither parent is purple, this CAN'T be an answer

A white-flowered plant is crossed with a pink-flowered plant.  All of the F1 offspring from the cross are white.

4. Which phenotype is dominant? white 
5. What are the genotypes of the original parent plants? WW (pure white) x ww (pink) 
6. What is the genotype of all the F1 offspring? Ww (white) 
7. What would be the percentages of genotypes & phenotypes if one of the white F1 plants is crossed with a pink-flowered plant? 

50% heterozygous  white & 50% homozygous recessive pink.

The cross for this question would be "Ww (white F1) x ww (pink)".
The alleles of the  white parent are above the columns & those of the pink parent are in front of the rows. 2 of 4 boxes (50%) are "Ww", which is heterozygous & would have the dominant trait (white).  The other 2 of 4 boxes (50%) are "ww", which is homozygous recessive & would have the recessive trait (pink).

8. Which of Mendel's Laws is/are illustrated in this question? Dominance is illustrated by the original cross (WW x ww).

9. Crossing two dihybrid organisms results in which phenotypic ratio?

A. 1:2:1 - genotype ratio of a hybrid cross, ex: Tt x Tt
B. 9:3:3:1- dihybrid means hybrid for two different traits. An example could be GgYy x GgYy.
C. 3:1 - phenotype ratio of a hybrid cross
D. 1:1

10. The outward appearance (gene expression) of a trait in an organism is referred to as:

A. genotype
B. phenotype
C. an allele
D. independent assortment

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11. In the homologous chromosomes shown in the diagram, which is a possible alleleic pair?

A. cD
B. Ee- a possible allelic pair but NOT SHOWN IN THE DIAGRAM, so this CAN'T be an answer
C. AB
D. ee - an "allelic pair" is always two forms of the same letter.  In this example they are two lowercase "e's".

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12. The phenotype of a pea plant can best be determined by:

A. analyzing its genes
B. looking at it
C. crossing it with a recessive plant
D. eating it

13. Mendel formulated his Law of Segregation after he had:

A. studied F1 offspring -
B. studied F2 offspring - he crossed two hybrids (F1's) and got a second generation --- the F2.
C. produced mutations - Mendel knew NOTHING about mutations so this CAN'T be an answer
D. produced hybrids

14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance?

A. TT x tt - one parent tall, the other short, all offspring would be tall
B. TT x Tt
C. Tt x Tt - illustrates Segregation
D. tt x tt

15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair.  This mating illustrates which of Mendel's Laws?

A. Dominance
B. Segregation
C. Independent Assortment - the question involves two different traits (hair color & hair texture), this is the only law that deals with two different traits
D. Sex-Linkage - Mendel knew NOTHING about sex-linkage so this CAN'T be an answer