5. 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?

Genes situated on the same chromosome are said to be linked

What is recombination?

Homologous chromosomes exchange by crossing over

How rare is recombination?

5-10% of all cells, few gametes contain recombinant genes

Crossover value equation

number of recombinants / number of progeny X 100%

Are genes more or less likely to recombinate if they are close together?

Less likely

Mutation definition

A change in the amount, arrangement or structure in the DNA of an organism

Which mutations can be inherited?

Only mutations in the gamete

How rare are mutations?

approximately 1 in 100,000 genes per generation

4 types of mutations

Gene/point - DNA not copied accurately, small amount of errors in bases
Chromosome mutations - get damaged or break, repaired but can be done incorrectly affecting a large range of genes

Aneuploidy - whole chromosome lost or added
Polyploidy - additional sets of chromosomes

External factors that cause mutations

Ionising radiation (UV, gamma and X)
Mutagenic chemicals (e.g. polycyclic hydrocarbons)

Different between mutagenic and carcinogenic

Mutagenic’s cause mutations, only some mutations are carcinogenic (case cancer)

2 main classifications of tumours

Benign - dosnt’t invade surrounding tissue or spread around body

Malignant - does ^, can lead to secondary tumours (metastases)

How do tumour suppressor genes work?

Slow down cell devision, repair DNA mistakes and tell cells when to die

What is a tumour?

a group of abnormal cells that may cause harm to the body, caused by a mutation

What do mutant p53 proteins do?

Don’t repair DNA and instead replicate it and mitosis occurs

What are oncogenes?

Mutated photo-oncogenes that trigger rapid cell division, allowing cells to divide out of control, causing cancer

Effect of tobacco

Tar collects in lungs, some chemicals get into nuclei of alveolar cells and slide between base pairs in the DNA, causing mutations

How does sickle cell anaemia occur?

Point mutation - the CTC triplet code (for glutamate) is substituted for CAC which codes for valine

What is non-disjunction?

A faulty spindle that means the chromosomes are not equally shared (1 cell gets 2 copies and the other gets 0)

How does downs syndrome occur?

A gamete with 2x trisomy 21 will fuse to another to produce an individual

What happens if there are an uneven number of chromosome sets in polyploids?

They are sterile becuase the chromosomes are unable to form as homologous pairs in meiosis