Component 2.5 - Inheritance

Question 1

What is a dominant allele?

Answer 1

An allele that is always expressed when it is present, whether it is homozygous or heterozygous

Question 2

What is a recessive allele?

Answer 2

An allele that is expressed only when it is homozygous

Question 3

What is an allele?

Answer 3

A different form of the same gene (same locus)

Question 4

What is the difference between co-dominance and incomplete dominance?

Answer 4

In co-dominance, both alleles of a gene are expressed. Shows the
phenotypes of both.

In incomplete dominance the phenotype is intermediate between that of the two homozgotes.

Question 5

What is a test cross?

Answer 5

The genotype being tested is crossed with a homozygous recessive to determine whether the parent had one or two dominant alleles.

If the F1 all have the dominant character the parent was homozygous dominant if 50% have it the parent was heterozygous.

Question 6

What suggests that a gene shows co-dominance or incomplete dominance?

Answer 6

If the phenotypes if offspring are in the ratio 1:2:1

Question 7

What are linked genes?

Answer 7

Those that are on the same chromosome and therefore do not segregate independently at meiosis and appear in the same gamete

Question 8

How do you recognise linkage?

Answer 8

If the results of crosses do not correspond with Mendelian ratios as Mendels laws only apply if genes are not linked

Question 9

How do you calculate the crossover value?

Answer 9

Number of recombinants/Number of individuals x100

Question 10

What if the calculated value of chi squared is less than the critical value?

Answer 10

This is equivalent to a probability greater than 5%, the null hypothesis is accepted at the 5% significance level and any difference is due to chance.

Question 11

What if the calculated value of chi squared is above the critical value?

Answer 11

It is equivalent to a probability less than 5%, the null hypothesis is rejected at the 5% significance level and there must be a different explanation for the differences in the data.

Question 12

Why are females the homogametic sex?

Answer 12

Because all the female’s secondary oocytes contain an X chromosome. The gametes are identical with respect to the X chromosome

Question 13

Why are males known as the heterogametic sex?

Answer 13

At meiosis 1. An X chromosome passes into one spermatocyte and a Y into the other. So the sperm cells contain half and half.

Question 14

How is the X chromosome different to the Y?

Answer 14

The X chromosome is much longer and only homologous with the Y at the tips

The Y also has the sex determining region which can switch on genes in other chromosomes

Question 15

Why are males more likely to be affected by recessive sex-linked conditions?

Answer 15

Because males only need one copy of the allele as they have only one X chromosome and no allele on the Y to be dominant over it

Females need two recessive alleles on their X chromosomes for it to be expressed.

Question 16

In a pedigree diagram what do the square and circle represent?

Answer 16

Square - male

Circle - female

Question 17

What is an autosome?

Answer 17

Any chromosome that is not a sex chromosome

Question 18

How can crossing over produce recombinants in linked genes?

Answer 18

1) When a cell with two linked genes e.g DEde undergoes meiosis there could be crossing over making 4 gamete types - DE, De, dE and de
2) DE and de would form with no crossing over but recombinants are combinations De and dE
3) Crossing over is rare so majority would still be DE and de but some other combinations would form mean the number gametes of gametes with each genotype are not equal so would not abide with Mendelian ratios