Inheritance

Question 1

What is a genotype

Answer 1

The genetic constitution of an organism (the genes and alleles an organism has)

Question 2

What is a phenotype

Answer 2

The expression of the genetic constitution and its interaction with the environment (set of characteristics an organism has)

Question 3

What is a gene

Answer 3

A length of DNA which codes for a particular protein occupying a specific locus on a chromosome

Question 4

What is an allele, with an example

Answer 4

Alternative forms of the same gene. Alleles of the same gene have very similar DNA base sequences
For example: gene is eye colour, alleles could be brown, blue or green

Question 5

What is a locus

Answer 5

The position of a gene on a chromosome

Question 6

What are the 2 types of alleles

Answer 6

Dominant and recessive

Question 7

What is a dominant allele and what is the effect on the phenotype

Answer 7

It’s a strong allele, and the effect of this allele will always be expressed in the phenotype

Question 8

How are dominant alleles displayed in exam questions

Answer 8

Upper case, e.g: A

Question 9

What is a recessive allele and what is the effect on the phenotype

Answer 9

It’s a weak allele and the effect of this allele is only expressed in the phenotype when there is no dominant allele present

Question 10

How are recessive alleles displayed in exam questions

Answer 10

Lower case, e.g: a

Question 11

In a diploid organism, how many alleles do each genotype have and where do they come from

Answer 11

In a diploid organism, each genotype has 2 alleles, as one allele is inherited from the female and one allele is inherited from the male at fertilisation

Question 12

Question: coat colour in mice, 2 alleles, black is dominant to brown
Let: B= black, b= brown
Name the genotype and phenotype of a homozygous dominant, heterozygous and homozygous recessive mouse.

Answer 12

Homozygous dominant
Genotype = BB
Phenotype = black
Heterozygous
Genotype = Bb
Phenotype = black
Homozygous recessive
Genotype = bb
Phenotype = brown

Question 13

What is a monohybrid cross

Answer 13

The cross which shows the inheritance of one characteristic

Question 14

What does pure breeding mean in genetics

Answer 14

They’re homozygous (e.g: BB or bb)

Question 15

When asked to work out the offspring in the F2 generation, what is the method to achieve the answer?

Answer 15

1. Always do a let (e.g: Let: B= black, b= brown)
2. Work out the phenotype, genotype and gametes of the PARENTS in the question
3. Do a punnet square of gametes to get genotype for F1 offspring
4. Work out the genotype, phenotype and ratio of the F1 offspring
5. Work out the phenotype genotype and gametes of the F1 parents ( to achieve F2 offspring)
6. Do a punnet square of gametes from F1 parents to get genotype for F2 offspring
7. Work out the genotype, phenotype and ratio of the F2 offspring.

Question 16

Question: work out the offspring in the F2 generation from a pure breeding black mouse and a pure breeding brown mouse

Answer 16

Let: B= black, b= brown
Parents
Phenotype: black. X. Brown
Genotype: BB. X. bb
Gametes: B. b

Fertilisation: B
b| Bb

F1 offspring
Genotype: Bb
Phenotype:Black
Ratio: All black

F1 parents
Phenotype: Black. X. Black
Genotype: Bb. Bb
Gametes: B b B b

Fertilisation: B. b
B| BB. Bb
b| Bb. bb
F2 offspring
Genotype: BB, 2Bb, bb
Phenotype: black black brown
Ratio: 3 : 1
(Black:brown)

Question 17

What is an important thing to remember about the genotype and phenotypes indicated in the offspring and what do you need to remember about each fertilisation with respect to the ratios

Answer 17

Genotypes and phenotypes indicated in the offspring are probabilities. Each fertilisation is a separate event ~ which male and female gametes combine at fertilisation is a random event. The chance of the ‘classic’ ratios being achieved is only likely to happen when there’s a large set of offspring to consider.

Question 18

What is a test cross

Answer 18

A breeding experiment to work out the genotype of an individual

Question 19

What do you have to do with a test cross

Answer 19

Breed the unknown (the one being worked out) with the known genotype (homozygous recessive)

Question 20

Draw a punnet square considering the offspring of a black mouse (Bb or BB) and a brown mouse (bb) and explain the findings of these punnet squares in a conclusion

Answer 20

Black mouse X. Brown mouse. Black mouse. X. Brown mouse
BB. bb. Or. Bb. bb
B. b. B. b. b
b. B. b
B|. Bb. b| Bb. bb

All offspring: Bb (black). Offspring: black : brown (1:1)

Conclusion: for any brown offspring, original parent which was black must be heterozygous

Question 21

What is a dihybrid cross

Answer 21

The inheritance of 2 characteristics at the same time

Question 22

Describe the method and outline who studied the shape and colour in peas and explain their conclusions from this study

Answer 22

Mendel studied the shape and colour in peas. He crossed a pure breeding (homozygous) round and yellow plant with a pure breeding (homozygous) green wrinkled plant. Found that all plants in the first generation were round and yellow.
Conclusions:
-round and yellow were dominant alleles in the 2nd generation
-round can be paired with green and yellow
-wrinkled can be paired with green and yellow
-independent segregation and crossing over of homologous chromosomes

Question 23

What was the ratio of yellow round: green round: yellow wrinkled: green wrinkled

Answer 23

YR: GR: YW: GW
9: 3: 3: 1

Question 24

Question: work out the offspring in the F2 generation from a pure breeding round yellow plant and a pure breeding wrinkled green plant

Answer 24

Let: R= round, Y= yellow, r= wrinkled, y= green
Parents F1 offspring
Phenotype: round yellow. X. Wrinkled green. Genotype: Rr Yy
Genotype: RR. YY. rr. yy Phenotype: round Yellow
Gametes: RY. ry Ratio: All round and yellow

Fertilisation: RY
ry| Rr. Yy

F2 offspring parents
Phenotype: round yellow. X. Round yellow
Genotype: Rr. Yy. Rr. Yy
Gametes: RY, Ry, rY, ry. RY, Ry, rY, ry
Fertilisation:
RY. | Ry. | rY. | ry
RY| RR YY | RR Yy | Rr YY |Rr Yy
Ry| RR Yy | RR yy | Rr Yy | Rr yy
rY | Rr Yy. | Rr Yy. | rr YY. | rr Yy
ry | Rr Yy. | Rr yy. | rr Yy. | rr yy
Ratio: round yellow: round green: wrinkled yellow: wrinkled green
9. : 3. : 3. : 1

Question 25

What is epistatis

Answer 25

It’s a type of dihybrid cross where one gene affects the expression of another gene

Question 26

What is epistatis the result of

Answer 26

The result of different enzymes catalysing different steps in a metabolic pathway, each step being dependent on a functional enzyme

Question 27

What is co dominance

Answer 27

When both alleles in heterozygous genotype are expressed. Both alleles are of equal dominance

Question 28

Why is a different terminology used for alleles in co dominance

Answer 28

Alleles are of equal dominance

Question 29

What is the different terminology for alleles in co dominance

Answer 29

E.g: red flowers = C^R White flowers = C^W

Question 30

Question: work out the F2 offspring from crossing a plant with red flowers with one with white flowers

Answer 30

Parents. F1 offspring Phenotype: red. x white. Genotype: C^rC^w Genotype: C^rC^r. C^wC^w. Phenotype: pink Gametes: C^r. C^w. Ratio: all pink Fertilisation: C^r C^w | C^rC^w F1 offspring parents. F2 offspring Phenotype: pink. X. Pink. Genotype: C^rC^r, C^rC^w, C^wC^w Genotype: C^rC^w. C^rC^w. Phenotype: Red. Pink. White Gametes: C^r. C^w. C^r. Ratio: 1. : 2. : 1 Fertilisation: C^r C^w C^r | C^rC^r. C^rC^w C^w |C^rC^w. C^wC^w

Question 31

What does it mean by the term multiple alleles

Answer 31

When one gene has more than 2 possible alleles

Question 32

What’s an example of multiple alleles

Answer 32

ABO blood group system in humans - the gene is the I gene and it has 3 possible alleles - the alleles are IA, IB, and IO - IA and IB are co dominant and both are dominant to IO (IO is recessive) E.g: Phenotype. Genotype A. IA IA or IA IO B. IB IB or IB IO AB. IA IB O. IO IO

Question 33

Question: work out possible blood groups of children from a mother with blood group A and father with blood group B. Assume both are heterozygous

Answer 33

Parents Phenotype: A. X. B Genotype: IA IO. IB IO Gametes. IA. IO. IB. IO Fertilisation: IB. IO IA. IA IB. IA IO IO. IB IO. IO IO F1 offspring Genotype: IA IB. IA IO. IB IO. IO IO Phenotype: AB. A. B. O Ratio: 1. : 1. : 1. : 1

Question 34

What is meant by co dominant alleles

Answer 34

If both alleles are present they’ll both be expressed

Question 35

What are sex chromosomes

Answer 35

They are X and Y and determine gender

Question 36

What’s the chromosome for female

Answer 36

XX

Question 37

What’s the chromosome for male

Answer 37

XY

Question 38

What’s the chromosomes for male and female in butterflies, birds and moths

Answer 38

XY = females XX = males

Question 39

What does sex linkage refer to

Answer 39

It refers to genes found on the sex chromosomes, X and Y

Question 40

Explain why the genes are usually on the X chromosome and what are Y chromosomes referred as

Answer 40

Usually genes on the X chromosome as there are few genes on the Y chromosome. The Y chromosome is referred as ‘genetically empty’

Question 41

Question: haemophilia is a sex linked condition on the X chromosome in which blood fails to clot properly. Let’s H = normal clotting h = haemophiliac What are the possible genotypes and phenotypes

Answer 41

XH XH = normal female XH Xh = normal female Xh Xh = haemophiliac female XH Y = normal male XH Y = haemophiliac male

Question 42

What does the normal female (XH Xh) show

Answer 42

This female is a carrier, she has the recessive allele but doesn’t express it

Question 43

Why are sex linked conditions more common in males than females

Answer 43

Males are XY and only have one X chromosome. Males only need to inherit one copy of the recessive allele to have a particular condition whereas females have 2 X chromosomes and must inherit 2 recessive alleles to be affected which is less likely to happen

Question 44

Question: red Green colourblindness is another sex linked condition on the X chromosome. Work out the offspring from a woman with normal vision (who is a carrier) and a man who is red green colourblind Let: B = normal vision b = colour blind

Answer 44

Parents Phenotype: normal female. X. Colourblind male Genotype: XB Xb. Xb Y Gametes: XB. Xb. Xb. Y Fertilisation: Xb. Y XB | XB Xb. XB Y Xb | Xb Xb. Xb Y Offspring Genotype: XB Xb, Xb Xb, XB Y, Xb Y Phenotype: normal female, colourblind female, normal male, colourblind male Ratio: 1. : 1. : 1. : 1

Question 45

What is autosomal linkage

Answer 45

Inheritance of genes that are located at different loci (parts) on the same chromosome. Genes are autosomes

Question 46

When can crossing over occur and why

Answer 46

Crossing over can occur when the homologous pairs are in a bivalent as genes are on the same chromosome

Question 47

When is crossing over less likely to occur and when is crossing over more likely to occur

Answer 47

Less likely to cross over: the closer 2 genes are to each other on the chromosome More likely to cross over: the further apart 2 genes are on the chromosome

Question 48

Question: work out the F2 offspring when a pure breeding fruit fly with grey body and long wings is crossed with black body and tiny wings. Assume no crossing over Let: L = long wings l = tiny wings G = grey body g = black body

Answer 48

Parents Phenotype: Grey long X. Black tiny Genotype: GG LL. gg ll Gametes: G|. |G. g|. |g L |. |L. l |. |l GL. gl Fertilisation: gl GL. Gg Ll F1 offspring Genotype: GgLl Phenotype: Grey long Ratio: all grey long F1 parents Phenotype: grey long. X. Grey long Genotype: Gg Ll. Gg Ll Gametes: G| | g. G| | g L | | l. L | | l GL. gl. GL. gl Fertilisation: GL. gl GL. GG LL. Gg Ll gl. Gg Ll. gg ll F2 offspring Genotype: GG LL, 2x GgLl, gg ll Phenotype: grey long, grey long, black tiny Ratio: 3. : 1