Inheritance flashcards

Topic 17: Inheritance

Continuous v discontinuous

Range v categories; genes and environment v genes only.

Structure of DNA (O Level)

Two strands coiled into a double helix. Each strand is a chain of nucleotides, each with a base (A, T, C or G). Bonds between pairs of bases hold the strands together: A with T, C with G.

In a DNA molecule 18% of bases are G. What percentage are A?

C = 18%, so A + T = 100 − 36 = 64%, and A = 32%.

How does DNA control the cell?

A gene is a length of DNA that codes for a protein. Its base sequence sets the amino acid sequence, which sets the protein's shape and function. DNA controls cell function by controlling protein production, including enzymes.

Allele

An alternative form of a gene.

Codominance

Both alleles in a heterozygote are expressed in the phenotype.

Phenotypic ratios from Tt × Tt and Tt × tt

Tt × Tt: 1 TT : 2 Tt : 1 tt, so 3 dominant : 1 recessive. Tt × tt: 1 Tt : 1 tt, so 1 dominant : 1 recessive.

Why do observed ratios often differ from expected ones?

Fertilisation is random, so a ratio is a probability. Small numbers of offspring often give ratios far from 3 : 1 or 1 : 1; larger samples come closer.

ABO blood group genotypes

A: IᴬIᴬ or IᴬIᵒ. B: IᴮIᴮ or IᴮIᵒ. AB: IᴬIᴮ. O: IᵒIᵒ. Iᴬ and Iᴮ are codominant; Iᵒ is recessive to both.

Gene mutation v chromosome mutation

Base sequence v chromosome number or structure.

Sickle cell anaemia and Down's syndrome

Sickle cell anaemia: a gene mutation in the haemoglobin gene changes the amino acid sequence and the haemoglobin shape. Down's syndrome: a chromosome mutation, 47 chromosomes instead of 46.

Natural v artificial selection

Environment selects v humans select.