Biology (university) · Part 07
Genetics and inheritance
How traits pass from parents to children, from Mendel's peas to linked genes and pedigrees.
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- AlleleOne version of a gene. An organism has two alleles for each gene, one from each parent. A dominant allele shows its effect even with one copy. A recessive allele shows only with two copies.
- Law of segregationMendel's first law. The two alleles for a gene separate when gametes form, so each gamete gets only one. A monohybrid cross between two heterozygotes (Aa x Aa) gives a 3:1 ratio of dominant to recessive in the offspring.
- Law of independent assortmentMendel's second law. Alleles of different genes are sorted into gametes independently, if the genes are on different chromosomes. A dihybrid cross of two double heterozygotes (AaBb x AaBb) gives a 9:3:3:1 ratio.
- Incomplete dominance and codominanceIn incomplete dominance the heterozygote looks like a blend of the two homozygotes. In codominance both alleles show fully and separately.
- Sex-linked inheritanceGenes on the X chromosome follow a different pattern in males (XY) and females (XX). A male has one X chromosome, so a recessive allele on it shows in him with no second copy to hide it.
- Linked genesGenes close together on the same chromosome tend to be inherited together. Crossing over can separate them. The further apart two genes are, the more often they recombine, so recombination frequency gives a map distance.
- PedigreeA family tree showing who has a trait. Squares are males and circles are females. Shaded symbols are affected. Pedigrees help work out whether a trait is dominant or recessive and autosomal or X-linked.
- Polygenic traitA trait controlled by many genes, often plus the environment. These traits vary continuously, giving a bell-shaped spread rather than clear categories.