2.4b Patterns of Inheritance

Question 1

Alleles

Answer 1

Different forms of a gene (e.g. gene = hair colour, alleles = brown, blonde.)

Question 2

Dominant

Answer 2

The allele which is dominant over the recessive allele (denoted by a
capital letter). It will mask the recessive allele e.g. a Hh individual will have the H phenotype.

Question 3

Recessive

Answer 3

The allele which is masked by the dominant allele (denoted by a lower case letter).

Question 4

Homozygous

Answer 4

When both alleles are the same in a genotype e.g. HH or hh.

Question 5

Heterozygous

Answer 5

When the alleles are different in a genotype e.g. Hh.

Question 6

Carrier

Answer 6

When an individual carries an allele but it does not show up in their phenotype e.g. in Hh the ‘h’ is carried.

Question 7

Genotype

Answer 7

The alleles that are present e.g. HH, Hh or hh.

Question 8

Phenotype

Answer 8

The physical characteristic determined by the genotype.

Question 9

Autosomes

Answer 9

The first 22 chromosomes.

Question 10

Sex Chromosome

Answer 10

The 23rd chromosome. XX = Female XY = Male.

Question 11

What is Autosomal Recessive Pattern of Inheritance caused by?

Answer 11

It is caused by the recessive allele therefore a sufferer needs 2 recessive alleles (hh). Therefore non-sufferers are HH and Hh.

Question 12

How can Autosomal Recessive Pattern of Inheritance be recognised?

Answer 12

You can recognise autosomal recessive from a family tree as is shows up
relatively rarely, can skip generations, affects males and females equally and two non-suffering parents can have a sufferer child (the parents would be Hh x Hb).

Question 13

How is Autosomal Dominant Pattern of Inheritance caused?

Answer 13

It is caused by the dominant allele therefore sufferers are HH and Hh. Therefore non-sufferers are hh.

Question 14

How is Autosomal Dominant Pattern of Inheritance recognised?

Answer 14

You can recognise autosomal dominant from a family tree as it shows up often
and does not skip generations, each sufferer has an affected parent and males and females are affected equally.

Question 15

Describe Autosomal Incomplete Dominance pattern of inheritance

Answer 15

This type of inheritance has no dominant or recessive allele. The 2 alleles have
incomplete (shared) dominance. Heterozygous individuals are part sufferers and have a mild form of the condition.
Non-sufferers have the genotype HH, part sufferers are HS and full sufferers are SS (if H is the normal allele and S is the disease causing allele).

Answer 16

You can recognise autosomal incomplete dominance from a family tree as full
sufferers occur rarely (as it can be deadly), part sufferers occur more frequently
and males and females are affected equally

Question 17

Females v Males Sex-linked Recessive pattern of inheritance

Answer 17

Females receive 2 copies of the gene and males only receive 1. This results in
many more male sufferers compared to female.
In females, non-sufferers have the genotype XHXH and XHXh (carrier) and sufferers have the genotype XhXh.
In males, non-sufferers have the genotype XHY and sufferers have the genotype XhY.

Question 18

How to recognise Sex-linked Recessive pattern of inheritance

Answer 18

You can recognise sex-linked recessive from a family tree as more males are
affected, fathers cannot pass the condition onto their sons and it occurs
relatively rarely (may skip generations).