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Learning goals
To outline Mendel’s cross breeding experiments and explain his contribution to our understanding of genetics
To define the following terms: traits, dominant, recessive, homozygous, heterozygous, genotype and phenotype
To predict possible outcomes of offspring traits using Punnett squares of simple dominant/recessive crosses
Inheriting traits from our parents
Our genetic instructions (genes) come from our parents. This is called heredity or inheritance. We know that chromosomes are made up of DNA and that genes are found within the DNA. Chromosomes and genes exist in pairs. Pair of genes, which are found at the same location on their respective chromosomes are called the locus of a gene.
Genes are sections of DNA that code for a particular protein which contributes to our characteristics, such as a gene for eye colour. The term allele describes the different forms of a gene. The combination of alleles for a gene inherited from the parents is known as the organism’s genotype. If two identical alleles are present in a person’s genotype, they are known as homozygous (or pure-breeding) for that trait, if there are two different alleles present then the person is known as heterozygous (or hybrid) for that trait.
For example, the trait of a widow’s peak. This trait is a single gene that is represented using two alleles: the allele of the dominant trait (presence widow’s peak) (assigned capital letter ‘H’ to show it is dominant) and the allele for the recessive trait (no widow’s peak) (assigned allele a lower case ‘h’ to show it is the recessive trait).
If you inherit the dominant allelle (H) from both parents, you are said to be homozygous dominant for that trait and your genotype will be HH. If you inherit the non-widow’s peak allele (h) from both parents, you are said to be homozygous recessive for that trait and your genotype will be hh.
If you inherit a widow’s peak allele (H) from one parent and a non-widow’s peak allele (h) from the other parent, you are heterozygous for that trait and your genotype will be Hh.
A trait or characteristic is described as dominant if you need only one allele for it to be expressed (HH or Hh, both of which result in the widow’s peak hairline).
For a recessive characteristic, the allele needs to be inherited from both parents in order for it to be expressed (hh, the normal non-widow's peak hairline is recessive).
Phenotype is the physical appearance. The ability to roll the tongue is the person’s phenotype for that trait. Always remember that genes and alleles refer to the genotype, and the physical appearance of the trait refers to the person’s phenotype.
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