Genetics (Chapter 16) Flashcards Preview

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What is a dominant allele?

One whose effect on the phenotype of a heterozygote is identical to its effect on a homozygote


What is a recessive allele?

One that is only expressed (in the phenotype) when no dominant allele is present


What are codominant alleles?

These both have an effect on the phenotype of a heterozygous organism


What is linkage?

The presence of two genes on the same chromosome so that they tend to be inherited together and do not assort independently


What is a test cross?

A genetic cross in which an organism showing a characteristic caused by a dominant allele is crossed with an organism that is homozygous recessive
- the phenotype of the offspring can be a guide to whether the first organism is homozygous or heterozygous


What is the F1 generation?

The offspring resulting from a cross between an organism with a homozygous dominant genotype, and one with a homozygous recessive genotype


What is the F2 generation?

The offspring resulting from a cross between two F1 (heterozygous) organisms


What is the phenotype?

The characteristics of an organism, often resulting from an interaction between its genotype and its environment


What is the genotype?

The alleles possessed by an organism


What does homozygous mean?

Having two identical alleles of a gene


What does heterozygous mean?

Having two different alleles of a gene


What is a mutation?

An unpredictable change in the genetic material of an organism


What is a gene mutation?

A change in the structure of a DNA molecule, producing a different allele of a gene


What is a chromosome mutation?

Mutations that cause changes in the structure or number of whole chromosomes in a cell


How do mutations occur?

Completely randomly (copying errors) or environmental factors e.g. ionising radiation or mutagens


What is a mutagen?

A substance that increases the chances of mutation occurring


What are the 3 different ways in which the sequence of bases in a gene may be altered during a gene mutation?

1) Base substitution
2) Base deletion
3) Base addition (insertion)


What happens during base substitution?

One base takes the place of another


What happens during base deletion?

One or more bases are lost from a sequence


What happens during base addition?

One or more extra bases are added to the sequence


Why do base additions/deletions usually have a very significant effect on the structure and therefore the function of the polypeptide that the allele codes for?

- Because they alter every set of 3 bases that follows them in the DNA molecule - they cause 'frame shifts' in the code
- Often the effects are so large that the protein made is totally useless
- Or, the addition/deletion may introduce a 'stop' triplet part way through a gene, so that a complete protein is never made


What is a silent mutation?

A mutation that has no apparent effect on an organism


Why do base substitutions often have no effect at all?

- They are often silent mutations because many amino acids have more than one triplet code, so even if one base is changed, the amino acid is still coded for
- They also have no effect on any of the triplet codes that follow, only the one that is directly affected


How could base substitutions have a very large effect?

If the substituted base makes the triplet a 'stop' triplet


What is one example of a base substitution that has a significant effect on the phenotype?

Sickle cell anaemia


Describe the substitution in sickle cell anaemia

- In people with the Hbs (sickle cell) allele, the base sequence CTT is replaced by CAT, changing one amino acid in the sequence which changes the beta-globin polypeptide


What is the effect of the small difference in the amino acid sequence in people with sickle cell anaemia?

- It makes little difference to the Hb molecule when combined with oxygen
- But when not combined with oxygen, the unusual beta-globin polypeptides make the Hb molecule much less soluble
- The molecules tend to stick to each other, forming long fibres inside the RBCs
- The RBCs are pulled out of shape, into a half-moon/sickle shape
- The distorted cell cannot carry oxygen and also gets stuck in capillaries, stopping any unaffected cells from getting through
- A person with this unusual beta-globin can suffer severe anaemia and may die


What is anaemia?

A lack of oxygen transported to the cells


What disease is an example of a relationship between a gene, enzyme and human phenotype?



Describe what happens in someone with albinism

- The dark pigment melanin is totally or partially missing from the eyes, skin and hair
- They have blue/pink irises, very pale skin and hair and poor vision

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