Simple inheritance patterns Learning objectives: inherited characteristics are controlled by genes, which have different forms, known as alleles. Inheritance is shown using genetic diagrams. inheritance of the sex chromosomes X and Y determines whether you are male or female understand the terms homozygous, heterozygous, genotype and phenotype.

Simple inheritance Which of these is dominant and recessive? Free ear lobe Which of these is dominant and recessive? Attached ear lobe Tongue-rolling Hitch-hiker’s thumb Dimples Handedness

Humans inherit two copies of each gene. Simple inheritance Humans inherit two copies of each gene. A different form or type of a gene is called an allele. Alleles that control a characteristic when there is only one copy are called dominant alleles. Recessive alleles are those that only control development if there are two copies – one on each chromosome – and no dominant allele. Because of meiosis, gametes have only one allele for each characteristic. The combination of alleles at fertilisation determines which trait the offspring inherits.

Simple inheritance Genetic diagrams can show the likelihood of inheriting a characteristic/trait: Punnett square Key terms: Homozygous Heterozygous Genotype Phenotype Father’s possible alleles Mother’s possible alleles

Simple inheritance Gender is determined by the sex chromosomes the child inherits Boys inherit one X and one Y chromosomes. Sperm cells have either X or Y. Girls inherit two X chromosomes. All egg cells have an X. Girl: XX Boy: XY Questions: What are the chances of having a boy or a girl? If a couple has a boy first, what will be the sex of their next child? Are there the same numbers of men and women in the population? If not, why do you think that is?

meiosis gene chromosome genotype DNA allele heterozygous homozygous Simple inheritance meiosis Key word SPLAT! gene chromosome genotype DNA allele heterozygous homozygous trait dominant phenotype Mendel gamete recessive Punnett square