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Biology · Genetic Inheritance

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Biology · AQA Paper 2

Genetic Inheritance

Genes, Alleles and Genetic Terms

Genotype plus environment gives phenotype, and a dominant allele shows even when only one copy is present.

  • Gamete A sex cell, such as a sperm cell or egg cell, that carries one copy of each gene.
  • Chromosome A long molecule of DNA in the nucleus that carries many genes.
  • Gene A small section of DNA on a chromosome that codes for a particular protein.
Three pairs of chromosomes showing the genotypes BB, Bb and bb and the fur colour each one gives.
The two alleles of a gene sit at the same position on a pair of chromosomes.

Homozygous or heterozygous?

  • Homozygous The two alleles are the same; BB (dominant) or bb (recessive); Only one kind of gamete can be made
  • Heterozygous The two alleles are different; Bb; Two kinds of gamete can be made, B and b

Single Gene Crosses and Punnett Squares

A Punnett square shows every possible combination of gametes, so it predicts how likely each outcome is, not what will actually happen.

  • Punnett square A diagram that shows all the possible combinations of gametes in a genetic cross.
  • Genetic cross A mating between two parents, used to predict the genotypes and phenotypes of the offspring.
  • Gamete A sex cell that carries one allele of each gene.
A Punnett square for a cross between two Bb mice, giving three black offspring for every one brown offspring.
Two heterozygous parents give a 3 to 1 ratio of the dominant to the recessive phenotype.

Two common crosses

  • Bb x Bb Gametes B, b and B, b; Offspring 1 BB : 2 Bb : 1 bb; 3 show the dominant phenotype to 1 recessive
  • Bb x bb Gametes B, b and b, b; Offspring 2 Bb : 2 bb; 1 dominant phenotype to 1 recessive

Completing a Punnett square

  • Write the parents' genotypes For example Bb and Bb, with B for black fur.
  • Write the gametes Each gamete has one allele from each parent.
  • Fill in the squares Copy the allele from the top and the allele from the side into…
  • Read off the phenotypes Use the rule that the dominant allele is expressed.
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Biology · AQA Paper 2

Genetic Inheritance continued

Inherited Disorders and Family Trees

Inherited disorders are caused by alleles, and whether the allele is dominant or recessive decides how the disorder moves through a family.

  • Inherited disorder A disorder caused by alleles that are passed from parents to offspring.
  • Polydactyly An inherited disorder in which a person has extra fingers or toes, caused by a dominant allele.
  • Cystic fibrosis An inherited disorder of cell membranes, caused by a recessive allele, that produces thick sticky mucus.
A family tree in which two unaffected parents have a daughter with a disorder and a grandson with the disorder.
Two unaffected parents with an affected child show that the allele is recessive.

Polydactyly and cystic fibrosis

  • Polydactyly Caused by a dominant allele; One copy is enough to have the disorder; An affected person must have an affected parent
  • Cystic fibrosis Caused by a recessive allele; Two copies are needed to have the disorder; Two unaffected carrier parents can have an affected child

Working out genotypes from a family tree

  • Decide if the allele is dominant or recessive Unaffected parents with an affected child means it is recessive.
  • Write the genotypes of affected people An affected person with a recessive disorder is nn.
  • Work out their parents Each parent passed on one n, and as they are unaffected they also have an N.
  • Use a Punnett square for the probability Cross the parents' genotypes and read off the chance for the next child.

Sex Determination

Every egg carries an X chromosome, so the father's sperm, carrying X or Y, decides the sex of the baby, with an equal chance of each.

  • Sex chromosomes The pair of chromosomes that determines the sex of a person: XX in females and XY in males.
  • X chromosome A sex chromosome carried by all eggs and by half of sperm.
  • Y chromosome The shorter sex chromosome, carried by half of sperm, which makes an embryo develop as male.
A Punnett square showing that half of children are XX girls and half are XY boys, beside the sex chromosomes of a female and a male cell.
Every egg carries an X chromosome, so the sperm decides the sex of the baby.

Female and male cells

  • Female (XX) Two X chromosomes in every body cell; Every egg carries one X; Cannot pass on a Y chromosome
  • Male (XY) One X and one Y in every body cell; Half of sperm carry X and half carry Y; Decides the sex of the baby

How the sex of a baby is decided

  • Eggs A woman is XX, so every egg cell she makes carries one X chromosome.
  • Sperm A man is XY, so half of his sperm carry an X chromosome and half…
  • Fertilisation An egg is fertilised by one sperm.
  • Chance Either kind of sperm is equally likely to reach the egg.