18 Genetics and Inheritance

Define Genotype

Collection of alleles (of genes) that an organism possesses

Genetic constitution of an organism

Define Phenotype

Expression of the genotype and its interaction with the environment

What does the phenotype include?

- Usually refers to the observable characteristics

- But it is anything expressed by the genotype

- This includes any protein produced eg blood type, enzymes etc

Define Gene

Section of DNA that codes for a polypeptide

Define alleles

Alternate versions of the same gene

Define Homozygous

A pair of chromosomes with the same alleles

Define Heterozygous

A pair of chromosomes with different alleles

Describe Dominant alleles

Always expressed in the phenotype if it is present in the genotype

Describe Recessive alleles

Only expressed in the phenotype if dominant allele is absent/ only expressed if homozygous

What 5 factors do we need to consider when investigating inheritance of specific genes/alleles

- Organism must show some discontinuous variation

- Must reproduce sexually

- It is pure breeding

- It has a short life cycle and produces large numbers of offspring

- Convenient to handle

Why must the organisms show some discontinuous variation?

- Allows you to clearly distinguish the characteristics tested as there will be no intermediates

Why must the organisms reproduce sexually?

Allows you to see the effect of recombining alleles and getting variation in offspring

Why must the organisms be pure breeding?

- Genotype of parents known as pure breeding are homozygous

Why must the organisms have a short life cycle and produce large numbers of offspring?

Allows lots of data to be collected in a short time and enough for stat analysis

Why must the organisms be convenient to handle?

Small, cheap, easy to keep/ feed

Define pure breeding

- Homozygous, same alleles

- When self fertilised, always produce identical offspring generation after generation (same phenotype)

Describe a Testcross / Backcross

- Determines genotype of an unknown organism (when you don't know if homozygous dominant or heterozygous)

- Cross the unknown with a recessive phenotype ( recessive is always homozygous)

- If any offspring have the homozygous recessive phenotype, the unknown must be heterozygous

- If all offspring have the dominant phenotype the unknown must be homozygous

Describe codominance

- When a gene has 2 alleles and neither can dominate the alleles are codominant

- So if both alleles are present in the genotype then both alleles are expressed in the phenotype

- This means a third phenotype arises with the heterozygous genotype - Both alleles are expressed in the phenotype (markscheme definition)

Describe the effect of multiple alleles being present

- Multiple alleles for the same gene eg red, white, blue

- Only 2 are present in the genotype

- There are more combinations possible

What is sex determined by?

- Determined by combination of chromosomes in a zygote rather than the genes themselves

- There are 2 types of sex chromosomes X (large) or Y (small)

- Human females have XX

- Human males have XY

Compare X and Y chromosomes

- Have homologous region which are present on both chromosomes

-Doesn't carry sex determining genes

- There is non homologous region which only X has

- Carries sex determining genes

What are sex linked genes

- Genes carried on the sex chromosomes but can be unrelated to the sexual characteristics

Describe the male sex chromosomes and their effect on sex linked disorders

- Non homologous X and Y chromosomes

- Many genes on X don't have equivalent on Y chromosome

- So recessive allele on X has more chance of being expressed in the phenotype

What must you remember when giving the phenotype of sex related conditions ( or similar )

Give the sex of the offspring

What are the 3 sex linked diseases you need to know?

- Haemophilia

- Colour blindness

- Muscular dystrophy

Describe Dihybrid Inheritance

- Considers how 2 genes coding for 2 different characteristics are inherited

- The characteristics are unrelated and assumed to be on different pairs of chromosomes

Describe Epistasis

- Form of dihybrid inheritance

- When the allele of one gene affects/ masks the expression of another in the phenotype

eg Gene for stopping hair production masks gene for blonde hair pigment - no hair for pigment to be added to

Describe Autosomal Linkage

- Non sex chromosomes are autosomes

- Genes on the same chromosomes are said to be linked

- They stay together during independent assortment in the first division of meiosis

- The alleles are passed onto offspring so fewer types of gametes will form

Describe crossing over

Exchange of alleles between chromatids of homologous chromosomes during the first division of meiosis ( meiosis 1 )

Describe the effect of crossing over on Autosomal Linkage

Use alleles GB (1 sets present) and gb (3 sets present) to explain

- Some gametes will have new allelic combinations

- Most offspring will be GgBb or ggbb

- Some new Gb and gB will form by crossing over

- So Ggbb and ggBb offspring will form

Describe the effect of distance between genes on likeliness of crossing over with Autosomal Linkage

- Closer together the genes on chromosomes, less likely that crossing over will result in recombinant genotypes

- The proportion of recombinants in the offspring can indicate the relative positions of the genes on a pair of chromosomes

- Smaller number means closer together