20.2

Question 1

Define codominance
E.g.?
What is the phenotypic ratio of codominant heterozygous cross?

Answer 1

occurs when two different alleles occur for a gene/
Both alleles are expressed, neither one is recessive
E.g. sickle cell anaemia and normal haemoglobin both dominant -people who are heterozygous have an in between phenotype called the sick- cell trait (have some normal haemoglobin and some sickle haemoglobin)

The phenotypic ratio is usually 1:2:1 (unaffected homozygous: heterozygous : disorder homozygous)

Question 2

Multiple allele crosses

Answer 2

Some genes have more than two versions, they have multiple alleles
E.g. blood group
I^A and I^B are codominant
I^O is recessive

I^O results in the production of neither antigen

Question 3

Describe dihybrid crosses
Expected phenotypic ratio?

Answer 3

Dihybrid inheritance is the inheritance of two characteristics, which are controlled b different genes

Dihybrid crosses can be used to show the likelihood of different genotypes for offspring
E.g. round/wrinkled yellow/green pea plants

Dihybrid cross between heterozygous parents gives phenotypic ratio of 9:3:3:1 (dominant both: dominant first, recessive second: recessive first, dominant second: recessive both)

Question 4

What are sex linked characteristics?

Answer 4

Characteristics coded by alleles located on a sex chromosomes

Question 5

Why are males more likely than females to express recessive phenotypes for genes that are sex linked

Answer 5

The Y chromosome is smaller than the X chromosome and carries fewer genes. So most genes on the sex chromosomes are carried on the X chromosomes (X-linked genes)

As males only have one X chronometer they often only have one allele for sex linked genes.

Females with one copy of the recessive allele won’t express it on their phenotype, but will be carriers for the X -link disorder
So because they only have only copy they express the characteristic of this allele even if its recessive, whereas females have another X chromosome which could mask the recessive allele (male- XY, fem- XX)

Question 6

E.g of genetic disorders caused by faulty genes on sex chromosomes

Answer 6

Colour blindness, haemophilia
The faulty alleles for both of these gene are on the X chromosome so are called X-linked chromosomes