Origins of genetic variation

Define variation.

The diversity of genetic and phenotypic traits within and between species.

It is a feature of sexually reproducing populations and the essential raw material for selection.

It gives species a greater opportunity to adapt to and survive in a dynamic environment.

What is phenotype influenced by?

Genotype and the environment

What does phenotypic plasticity allow an organism to do, and how does this help them?

It allows their phenotype to alter in response to their environment.

It helps give the organism a greater chance of survival in ever-changing surroundings.

What is the monohybrid cross?

A simple breeding experiment involving just a single characteristic.

What did Gregor Mendel produce and what does it do?

The test cross.

It creates a way to establish the genotype of an organism with the dominant phenotype for a particular trait.

How do you perform a test cross and what is it testing?

It is used to find out if something which has a dominant phenotype is heterozygous or homozygous.

It involves the individual with the unknown genotype and known dominant phenotype being bred with a homozygous recessive individual. The homozygous recessive can only produce only one type of allele (recessive), so the phenotypes of the offspring will reveal the genotype of the unknown parent.

If the unknown individual is homozygous dominant for the trait, all of the offspring will display the dominant phenotype. But if the offspring display both dominant and recessive phenotypes the unknown must be heterozygous for the trait.

What organisms is often used to illustrate basic principles of inheritance and why?

The common fruit fly.

Because they have a wide range of heritable mutations. And many genetic markers which can easily be identified e.g. body colour. They also produce large numbers of offspring and have a relatively short reproduction cycle. And are low maintenance so they are ideal for studying in the lab.

What is Mendel’s First Law (the principle of segregation)?

Individuals carry two discrete hereditary factors (alleles) controlling each characteristic. The two alleles segregate (or separate) during meiosis, so each gamete carries only one of the two alleles.

This can be explained by the behaviour of homologous chromosomes during meiosis.

What are the two ways the results of a genetic cross can be displayed?

1. Punnett square

2. Pedigree diagram (like a family tree)

What does a pedigree diagram show?

The inheritance of a particular characteristic through a family, they are most often used for humans, but are also used for commercial animals such as racing horses or pedigree dogs.

What is a gene?

A length of DNA that codes for a protein.

How does genotype control phenotype?

The gene is a length of DNA that codes for a protein.

It is the proteins that control phenotype in their roles as enzymes, pumps, transporters, hormones, structural elements etc.

Phenotype is also influenced by the environment.

What are the different types of chromosomes?

• Homozygous dominant (when both chromosomes have identical copies of the dominant allele)

• Homozygous recessive (when both chromosomes have identical copies of the recessive allele)

• Heterozygous (when the chromosome has different copies of alleles)

What is sex linkage?

A special case of linkage when a gene is located on a sex chromosome (usually the X), this means that the character encoded by the gene is usually seen only in one sex (the heterogametic sex [XY]) and occurs rarely in the homogametic sex.

Sex linkage genes are responsible for a number of heritable disorders in males, women who have the recessive alleles on their chromosomes are said to be carriers.

What is codominance?

It is an inheritance pattern in which both alleles in a heterozygote contribute to the phenotype.

Both alleles are independently and equally expressed.

E.g. human blood group AB is the result of two alleles: A and B, both being equally expressed. Or certain coat colours in horses and cattle, reddish coat colour is equally dominant with white so animals with both alleles have coats that are roan (with a mix of red and white hairs). The red hairs and white hairs are expressed equally and independently (not blended to produce pink).

What is sickle cell anaemia and what is it an example of?

It is an example of codominance.

There are three phenotypes

Hb^A Hb^A Normal: all haemoglobin molecules are normal, with normal disk-shaped red blood cells.

Hb^s Hb^s Sickle cell anaemia: all haemoglobin molecules are abnormal, so most red blood cells are sickle-shaped. So they are less flexible than normal cells, so can block capillaries and arterioles, causing death and severe pain. Sickle cells are also destroyed by the spleen faster than they can be made, so not enough oxygen can be carried in the blood (anaemia). Without treatment this phenotype is fatal in early childhood.

Hb^A Hb^s Sickle cell trait: 50% of the haemoglobin molecules in every red blood cell are normal and 50% abnormal. Long chains do not form, so the red blood cells are normal and carry oxygen normally. However, these red blood cells do sickle when infected by the malaria parasite, so infected cells are destroyed by the spleen. Therefore, this phenotype provides immunity for malaria, and is common in areas of the world where malaria is endemic.