Topic 8.1 - Inheritance

Question 1

Define gene

Answer 1

A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic

Question 2

Define Genotype

Answer 2

All of the genes within an organism and their alleles

Question 3

Define Phenotype

Answer 3

The expression of the genetic constitution and its interaction with the environment

(i.e. an organism’s characteristics)

Question 4

Define Allele

Answer 4

A different version of a gene

Question 5

Explain what is meant by a recessive allele

Answer 5

Only expressed in phenotype when 2 alleles are present

Question 6

Explain what is meant by a dominant allele

Answer 6

Is always expressed in the phenotype

Question 7

Explain what is meant by co-dominant alleles

Answer 7

Both alleles are expressed in the phenotype

Question 8

Define Homozygous

Answer 8

An organism that carries 2 copies of the same allele

Question 9

Define Heterozygous

Answer 9

An organism that carriers 2 different alleles for a gene

Question 10

Define Carrier

Answer 10

Person carrying an allele which isn’t expressed in the phenotype but can be passed on to offspring

Question 11

Define Locus

Answer 11

Fixed position of gene on a chromosome

(Alleles of gene are found at same locus on each chromosome in a pair)

Question 12

Define Autosome

Answer 12

Any chromosome that is not a sex chromosome

Question 13

Define Allosome

Answer 13

Sex chromosome

Question 14

What is monohybrid inheritance?

Answer 14

Inheritance of a characteristic controlled by a single gene

Question 15

Draw a monohybrid cross showing how wing length is inherited in fruit flies when the parent’s genotypes are NN x nn. Include the phenotypes and phenotypic ratio.

N - normal wings allele

n - vestigial (little) wings allele

Answer 15

Question 16

In the ABO blood group system in humans there are 3 alleles for blood type:

• I(O) is the allele for blood group O
• I(A) is the allele for blood group A
• I(B) is the allele for blood group B

Allele I(O) is recessive. Alleles I(A) and I(B) are codominant - people with genotype I(AB) will have blood group AB.

Draw a genetic diagram that shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B.

Answer 16

Question 17

What do dihybrid crosses show?

Answer 17

Show how 2 genes are inherited are once

Question 18

Draw a dihybrid cross diagram for when the both parents’ gentotype is RrYy. Include the phenotype and the phenotypic ratio.

• R - round seed
• r - wrinkled seed
• Y - yellow seed
• y - green seed

Answer 18

Question 19

Answer 19

Question 20

When are characteristics said to be sex-linked?

Answer 20

When the allele that codes for a characteristic is located on a sex chromosome - particularly the part of an X chromosome that is missing in a Y chromosome

Question 21

Most genes on sex chromosomes are only carried on ___ chromosomes

Answer 21

X

(X-linked genes)

Question 22

Explain why males are more likely than females to show recessive phenotypes for genes that are sex-linked

Answer 22

• Males only have 1 X chromosome so only have 1 allele for sex-linked genes
• because they have only 1 copy, they express the characteristic of the allele even if its recessive

Question 23

Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome

Given that the female parent is a carrier and the male is unaffected, draw monohybrid diagram (n = faulty allele for a colour vision)

Answer 23

Question 24

What are autosomal genes?

Answer 24

Genes located on the autosomes

Question 25

Genes on the same autosome are said to be ____

Answer 25

linked

Question 26

Why are autosomal genes said to be linked?

Answer 26

• because they’re on the same autosome, they’ll stay together in independent segregation of chromosomes in meiosis I
• And their alleles will be passed on to offspring together

(Only doesn’t occur if crossing over splits them up 1st)

Question 27

The closer together 2 genes are on autosome, the more …

Answer 27

closely they’re linked

Question 28

Why is that the closer together 2 genes are on the autosome, the more closely they’re linked?

Answer 28

because it reduces the chance of genes being separated during crossing over

Question 29

What happens to the phenotypic ratio expected in offspring when 2 gene are autosomally linked?

Answer 29

• It changes - won’t get the phenotypic ratio you expect in offspring of cross
  
  • e.g. Dihybrid cross between 2 heterozygous parents = 9:3:3:1 ratio in offspring
  • Instead phenotypic ratio more likely to be like monohybrid cross between heterozygous parents (3:1)

Question 30

Why is the phenotypic ratio different when 2 genes are autosomally linked?

Answer 30

because there is a higher proportion of offspring with their parents’ (heterozygous) genotype and phenotype

Question 31

Explain the difference in the phenotypic ratios

Answer 31

• NR and nr linked (in NnRr parent)
• NnRr parent produces mainly NR and nr gametes
• So fewer Nnrr and nnRr
• Crossing over produces some Nr and nR gametes

Question 32

What is epistasis?

Answer 32

When an allele of 1 gene masks (blocks) the expression of the alleles of other genes

Question 33

Name the 3 types of epistasis

Answer 33

• Recessive epistasis
• Dominant epistasis
• Complementary epistasis

Question 34

Describe dominant epistasis

Answer 34

Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene

Question 35

Dominant Epistasis

State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent

Answer 35

12 : 3 : 1

dominant epistatic : recessive epistatic dominant other : recessive both

Question 36

Squash colour is controlled by 2 genes - the colour espistatic gene (W/w) and the yellow gene (Y/y). The no-colour, white allele (W) is dominant over the coloured allele (w). The yellow gene has the dominant yellow allele (Y) and the recessive green allele (y).

Draw a genetic digram crossing WwYy with WwYy. Include the phenotypes and phenotypic ratio.

Answer 36

Question 37

Describe recessive epistasis

Answer 37

Having 2 copies of the recessive epistatic allele masks (blocks) the expression of the other gene

Question 38

Recessive Epistasis

State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent

Answer 38

9 : 3 : 4

dominant both : dominant epistatic recessive other : recessive epistatic

Question 39

Flower pigment in a plant is controlled by 2 genes. Gene 1 codes for a yellow pigment (Y = dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R = dominant yellow allele). Cross YyRr x YyRr. Include phenotypes and phenotypic ratios.

Answer 39

Question 40

When do you use the chi-squared (χ2) test?

Answer 40

Used to see if results of experiment support a theory. Only for catergorical data - eg. phenotypes observed.

Question 41

State the chi-squared formula

Answer 41

• O = Observed result
• E = Expected result

Question 42

To find if there’s a significant difference between your observed and expected results, you need to compare χ2 value to the ____ ____

Answer 42

critical value

Question 43

If χ2 ≥ critical value, there’s a

Answer 43

• Significant difference
• (Something other than chance causing difference)
• Null hypothesis can be rejected

Question 44

If χ2 < critical value, there’s …

Answer 44

• NO significant difference
• Accept null hypothesis

Question 45

Describe how you work out the degrees of freedom

Answer 45

Number of classes (no. of phenotypes) - 1

e.g. 2 - 1 = 1

Question 46

How do multiple alleles of a gene arise? (2)

Answer 46

• Due to mutations
• Which occur in different positions in the gene

Question 47

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive (2)

Answer 47

• 3 & 4 produce 9/11 who are affected
• Both 3 and 4 are heterozygous (carry recessive allele)

Question 48

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome (2)

Answer 48

• 11 is affected, 3 is not
• (If on X) 11 / affected female would not receive the recessive allele on X chromosome / X^t from 3 / father

Question 49

Explain why sex-linked conditions are more common in males than in females (3)

Answer 49

• Males have XY chromosomes and females have XX chromosomes
• Recessive allele on X chromosome has no (equivalent) allele on Y chromosome
• Male needs to inherit one recessive allele whereas female needs to inherit two

Question 50

Suggest one reason why observed ratios are often not the same as expected ratios (1)

Answer 50

• Due to chance
• Fertilisation is random

Question 51

How is variation genetically controlled? (1)

Answer 51

Polygenes / several genes

Question 52

Extension: State the genotype of the heterozygous parent using the correct notation

Answer 52