Inheritance

Question 1

Define genotype.

Answer 1

The genetic constitution of an organism.

(eg.BB, Bb, bb for eye colour)

Question 2

Define phenotype.

Answer 2

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

(eg. brown eyes)

Question 3

Define allele.

Answer 3

Different versions of the same gene.

Question 4

Define dominant.

Answer 4

The dominant allele is always expressed.

(eg. BB = blue eyes / Bb = blue eyes)

Question 5

Define recessive.

Answer 5

The allele is only expressed if it is homozygous recessive.

(eg. bb = brown eyes)

Question 6

Define co-dominant.

Answer 6

Both alleles are expressed in the phenotype.

eg. (using superscripts)
C^R C^R = red flowers
C^W C^W = white flowers
C^R C^W = pink flowers

Question 7

Define homozygous.

Answer 7

Both alleles at a specific locus on each homologous chromosome are the same.

BB - homozygous dominant
bb - homozygous recessive

Question 8

Define heterozygous.

Answer 8

Both alleles at a specific locus on each chromosome are different.

Question 9

Define monogenic inheritance.

Answer 9

The characteristic inherited is controlled by a single gene.

Question 10

Define dihybrid inheritance.

Answer 10

The inheritence of two different characteristics controlled by two different genes (on different chromosomes).

Eg. pure breeding pea seeds
- Yellow and round = YYRR (both dominant)
- Green and wrinkled = yyrr (both recessive)

Any of these alleles can combine due to independent segregation.

Draw a 2x2 punnet square of parent genotypes to find gametes.
SO possible gametes:
- YR, YR, YR, YR
- yr, yr, yr, yr

THEN draw 4x4 punnet square to find offspring genotypes:
16 - YyRr

THEN work out the offspring phenotypes:
YyRr = yellow and round

Question 11

What does a dihybrid cross of pure breeding seeds result in?

Answer 11

All the F1 generation is heterozygous for both seed colour and shape.

Question 12

What is the phenotypic ratio for the F2 generation of pure breeding seeds from a dihybrid cross?

Answer 12

9 : 3 : 3 : 1

Question 13

Define multiple alleles.

Answer 13

3 or more possible alleles for one individual trait within a population.

Eg.
Blood groups = A /B / AB / O
I^A and I^B are co-dominant
I^O is recessive

Question 14

Define autosomal linkage.

Answer 14

2+ genes which are located on the same autosome

(ie. on the same single non-sex chromosome)

Question 15

What happens to genes that are autosomally linked during meiosis I?

Answer 15

• They stay together during independent segregation.
• There is no crossing over so linked genes stay together.
• Linked genes pass onto gametes and offspring together.
• A higher proportion of offspring will have the parents’ genotype and phenotype as the autosomally linked genes are inherited together.

Question 16

What is the phenotypic ration for autosomal linkage?

Answer 16

3:1

(ALWAYS)

Question 17

Define sex-linked.

Answer 17

A gene whose locus is on a sex chromosome.

Question 18

Why are genes more likely to be linked to the X chromosome?

Answer 18

• The X chromosome is larger.
• Some genes are found on sections on the X chromosome which are not present on the Y chromosome.

Question 19

What is the genotype for males and females?

Answer 19

Males = XY
Females = XX

Question 20

Why do more males have sex-linked disorder?

Answer 20

• Males = XY
• The can’t be heterozygous.
• SO they will always express the recessive X-linked allele if one copy is present.

Question 21

Define epistasis.

Answer 21

The interaction of non-linked genes where one masks the expression of the other.

‘epi’ = on top of
‘stasis’ = standing

Question 22

What is the chi-squared (X^2) test?

Answer 22

A statistical test used to find out whether the difference between the observed vs expected data is due to chance.

Question 23

What is the limitation of a chi-squared test? And what can be done to combat this?

Answer 23

• The sample may be unrepresentative of the whole population.
• Use a larger sample size / statistical analysis

Question 24

When do we use the chi-squared test?

Answer 24

• Categorical data
• Absolute numbers in data

Question 25

What does it mean if the calculated value of chi-squared is larger than the critical value at P = 0.05?

Answer 25

• The results are significant.
• There is less than 5% probability that the differences were due to chance.
• Reject the null hypothesis.

Question 26

What does it mean if the calculated value of chi-squared is smaller than the critical value at P = 0.05?