Inheritance

Gene definition

A sequence of DNA that codes for a polypeptide which occupies a specific locus

What can genes do? (3)

• separate and combine

• mutate

• code for specific polypeptides

Allele defintion

A variant nucleotide sequence found at a specific locus which codes for an altered phenotype

Dominant

the allele in a monohybrid cross that always produces an effect on the phenotype of the organism when present, usually represented by a capital letter

Recessive

The allele that produces an effect on the phenotype only when present as an identical pair, often lower case

Genotype

The combination of alleles found in an individual

Phenotype

The appearance of an organism - it is determined by the genotype

Homozygous

If both alleles of a gene are the same, e.g. TT, tt.

Heterozygous

The pair of alleles of a gene are different, e.g. Tt.

F1

first filial generation - of offspring from a genetic cross

F2

second filial generation - of offspring from a genetic cross

Homologous

A pair of chromosomes - one of which has come from the mother, the other from the father, can then form a bivalent during prophase of meiosis I.

Autosomal chromosomes

Chromosomes which do not control the sex of an individual

Sex chromosomes

These carry the genes that control whether an individual develops male or female reproductive systems

Mendel’s 1st law

The characteristics of an organism are determined by alleles which occur in pairs. Only one of the pair can be present in a single gamete

Monohybrid inheritance

The inheritance of one characteristic which is controlled by single genes and genes on different chromosomes. Involves the inheritance of two alleles of the same gene. An example of discontinuous variation.

Dihybrid inheritance

The inheritance of two separate genes

Mendel’s 2nd law

Each member of an allelic pair may combine randomly with either of another pair

What is a test cross?

Crossing an organism with the dominant phenotype with one that has the recessive phenotype to determine if the characteristics are due to one or two dominant alleles

What happens in codominance?

Both alleles are expressed individually in a zygote/phenotype so the heterozygote offspring has a combination of both characteristics

What is a zygote?

A fertilised egg cell

3 important mendelian ratios

3:1 - dominant/recessive, heterozygous parents

9:3:3:1 - dominant/recessive, 2 genes, both heterozygous parents

1:2:1 - incomplete dominance, 2 heterozygous parents

Why are statistical tests used?

To decide if the data from an experiment fits any Mendelian ratios

How is sex determined?

X chromosomes from egg/sperm

Y chromosomes from sperm

What is the generic null hypothesis for inheritance?

There is no significant difference between the observed and expected, any deviation is due to chance

How do you calculate critical value?

Degrees of freedom is always one less than the number of classes of data

Always use the 0.05 (5%) column

What is linkage?

Somatic cells contain 46 chromosomes, humans have thousands of characteristics so each chromosome must carry a large number of genes

Genes situation of the same chromosome are said to be linked