D.3.2 - Inheritance

Chromosome pairs (identical)

Each chromosome pair has two identical chromosomes containing the same genes, meaning humans have two copies of each gene they have

Allele

Versions of genes (gene = eye colour, allele = green, brown, blue)

Gene expression (molecular reaction)

Creation of a protein

Homologous pair

A pair of chromosomes that have the same genes

Dominant gene

The gene that determines which trait is given

Recessive gene

Genes that are expressed but are not given if dominated

Homozygote

Both alleles are dominant or recessive (BB or bb)

Heterozygote

One gene is dominant or recessive (Bb)

Genotype

Combination of genes/alleles

Phenotype

The physical appearance of the genotype 

Sex-linked traits exp.

• Colour blindness: humans

• Duchenne muscular dystrophy: humans

• White eye colour: fruit flies

• Tortoiseshell/calico fur: cats

Why can only women be carriers for e.g haemophilia?

It is a sex-linked recessive allele, so men only have one version of the allele which must either be dominant or recessive (since they only have one X-chromosome) but women can have both since they have two X-chromosomes.

Polygenic inheritance 

Two or more genes influencing the expression of a single trait. 

How to tell if a gene is autosomal or X-linked?

If it is passed from father to son, it cannot be X-linked as the father always passes on the Y chromosome to his sons

PKU inheritance

Two parents who are heterozygous carriers for PKU have a 25% chance of having a child with it because it is a recessive alllele

What is PKU

Autosomal, caused by a mutation in the PAH gene, results in low levels of the enzyme phenylalanine hydroxylase, which causes rise in the level of phenylalanine - this can impair brain development in children. 

Autosomal recessive diseases exp.

• PKU

• Albinism

• Cystic fibrosis

• Sickle cell anaemia

• Tay Sachs disease

• Thalassemia

Pedigree how to tell if a gene is recesisve or dominant

If it is dominant, it will be present in every generation