17.1 Studying inheritance

Question 1

What is a genotype

Answer 1

It is the genetic make up of an organism.
It describes all the alleles that an organism has

Question 2

What is a phenotype

Answer 2

. The observable or biochemical characteristics of an organism
. It is the result of the interaction between the expression of the genotype and the environment

So the environment can alter the phenotype of an organism

Question 3

What is a gene

Answer 3

A length of DNA , which is a sequence of nucleotide bases, that normally code for a particular polypeptide

Question 4

What is an allele

Answer 4

A different form of a gene, so one gene may have multiple alleles

For example, in pea plants, there is a gene for the colour of the seed pod, and this gene has two forms: allele for green pod and allele for yellow pod

Question 5

What is a locus

Answer 5

The position of a gene on a particular DNA molecule

Only one allele of a gene can occur at the locus of any one chromosome

Question 6

What are homologous chromosomes

Answer 6

Chromosome pairs, one from the mother and one from the father
They code for the same genes with the same corresponding loci

So there are two loci that each carry one allele of a gene

Question 7

What is homozygous

What about heterozygous

Answer 7

There are two loci that each carry one allele of a gene.

If the allele on each of the chromosomes is the same, the organism is homozygous for the character
Eg both alleles for green pods are present

If the alleles are different, the organism is heterozygous
Eg if one chromosome has an allele for green pods and the other has an allele for yellow pods

Question 8

Usually, when two alleles are present in the genotype, only one of them shows itself in the phenotype

What is a dominant allele

What is a recessive allele

Answer 8

The allele of the heterozygote that always expresses itself in the phenotype is dominant. So only one is needed to be expressed.
If two dominant alleles are present, it is homozygous dominant

Recessive means two of the same identical allele need to be present for that phenotype to be expressed
If two recessive alleles are expressed it is homozygous recessive

Question 9

What does co-dominant mean

Answer 9

Two alleles both contribute to the phenotype

So when both alleles occur together, the phenotype is either a blend of both features
(eg shorthorn cattle with roan coat colour from allele for red hair and allele for white hairs)
or both features are represented (eg the presence of both A and B antigens in blood group AB

Question 10

What is the cause of an organism having multiple alleles

Answer 10

When a gene has more than two allelic forms, there can be multiple alleles for a characteristic.

In a homologous pair, there are always two chromosomes so only two of the 3 or more alleles in existence can be present in a single organism