SEX CHROMOSOMES

Genetics and Genetic Engineering

Sex chromosomes

Chromosomes which determine sex of an organism and carry sex-linked genes

Describe characteristics of sex chromosomes

• Referred to as gonosomes and make up the 23rd pair in humans

• Longer X chromosome and shorter Y chromosome

Describe the overall chromosome composition in gametes

• Biological females - 22 + X - 22 autosomes and one X gonosomes

• Biological males - 22 + X OR 22 + Y- 22 autosomes and one X gonosome OR 22 autosomes and one Y gonosome

Outline the inheritance of sex-linked genes in males

• Males only have one X chromosome so certain genetic disorders occur more often in males than females

• If abnormal allele on X chromosome of male, they will have genetic disorder

Outline the inheritance of sex-linked genes in females

• Females have two X chromosomes so have a much smaller chance having a sex-linked disorder

• If they carry abnormal recessive allele on one X chromosome and normal dominant allele on other X chromosome she will not have genetic disorder

• If they carry abnormal recessive allele on both X chromosomes she will have genetic disorder

Red-green colour blindness

A visual defect resulting in an inability to distinguish between certain colours

Outline the inheritance of red-green colour blindness

• Gene for colour blindness is recessive and carried on X chromosome

• Therefore sex-linked/X-linked recessive

Haemophilia

A condition where blood does not clot properly and takes a long time to clot due to the lack of an important clotting factor

Outline the inheritance of haemophilia

• Gene for colour blindness is recessive and carried on X chromosome

• Therefore sex-linked/X-linked recessive

Polygenic characteristic

A hereditary characteristic controlled by more than one gene pair at different loci

Describe how polygenic characteristics have greater diversity of phenotypes

• More alleles control a specific characteristic so more possible gene combinations - does not follow Mendalian inheritance

• Series of phenotypes usually occur over a continuous spectrum, ranging from one extreme to another - known as continuous variation

What is skin tone an example of

Polygenic inheritance

Describe skin tone as an example of polygenic inheritance

• The broad spectrum of skin colours does follow Mendalian inheritance (darker skin vs lighter skin)

• BUT variation in skin tones in broad spectrum of skin colours (variation of tones in darker skin and the variation of tones in lighter skin) is polygenic