A2 AQA Biology Unit 4 Inheritance

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Genes and Phenotypes

A gene is a length of DNA that codes for the production of a particular protien

Genes affect the characteristics of an organism, known as its phenotype. Environment can also affect phenotype.

A diploid cell contains two sets of chromosomes. There are therfore two copies of each type of chromosome, and two copies of the genes that they carry.

Homoglous chromosomes carry the same genes at the same loci.

Genes often come in diffeent forms, called alleles.

The alleles of a gene that an organism has are known as its genotype. If the two alleles are the same, it is homozygous. If they are different, it is hetrozygous.

An allele that has its full effect on the phenotype even when a different allele of the same gene is present is said to be dominant. An allele that only has its effect when no other allele is present is said to be recessive. If both alleles have an effect in a heterozygote, they are said to be codominant.

Some genes have three or more different alleles, known as multiple alleles

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Inheriting Genes

Gametes are haploid, and therefore contain only one copy of each gene. If a person is homozygous, then the genotype of all the gametes is the same. If they are heterozygous, then half of thier gametes contain the other allele.

There is an equal chance of any gamete from one parent fusing with any gamete from the other. The genetic diagram shows the different genotypes that can arise in the offspring, and the relative chances of the genotype occuring.

Offspring ratios predicted by genetic diagrams are only probabilities, and the actually results may not be ezactly the same, especially if small numbers are involved.

In a sitution involng dominance, if two heterzygous organisms are crossed we could expect a 3:1 ratio of offspring showing the dominant characteristic to those showing the recessive characteristic in thier phenotype. If a heterozygous organism was crossed with a homozygous recessive one, then we would expect a 1:1 ratio in the offspring

in humans, sex is determined by the X and Y chromosomes. Genotype XX produces females, and XY produces males

Numerous genes that are found on the X chromosome are not found on the Y chromosome. These are called sex linked genes.

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Genes in Populations

The gene pool is all the alleles of all the genes that are present in a population

A population is a group of organisms of the same species that can interbreed. A species may contain many different populations, which can interbreed within thier population but not with other populations.

The Hardy-Weinburg equation allows us to calcuate the frequency of a particular allele in a population, assuming that the allele frequencies do not change from generation to generation.It is p^2 +2pq + q^2 = 1, where p^2 is the frequency of the homozygous dominant genotpe (for example, FF) 2pq is the frequency of the heterozygous genotype (Ff) and q^2 is the frequency of the homozygous recesive genotype (ff)

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