Inherited change

Question 1

What is a Genotype

Answer 1

the genetic constitution of an organism and it describes all the alleles that an organism has
- the genotype determines the limits within which the characteristics of an individual may vary

Question 2

What is a Phenotype

Answer 2

the biochemical characteristics of an organism.
It is the result of the interaction between the expression of the genotype and the environment.
The environment can alter an organisms phenotype

Question 3

What is a gene?

Answer 3

a length of DNA that normally code for a particular polypeptide

• the polypeptide may be an enzyme that is needed in the biochemical pathway that leads to the production of the characteristic
• genes exist in two or more forms called alleles
• the position of a gene on a particular DNA molecule is known as the LOCUS

Question 4

What is an allele?

Answer 4

one of the different forms of a gene

- only one allele of a gene can occur at the locus of any one chromosomes

Question 5

homologous chromosomes

Answer 5

chromosome pairs one from each parent that are similar in length, gene position and centromere location

Question 6

define homozygous

Answer 6

having two identical alleles of a particular gene

Question 7

define hetrozygous

Answer 7

having two different alleles of a particular gene

Question 8

Dominant and Recessive allele

Answer 8

The allele of the hetrozygote that expresses itself in the phenotype is said to be dominant whereas while the one that is not expressed is said to recessive

Question 9

homozygous dominant

Answer 9

a homozygous organism with two dominant alleles

Question 10

homozygous recessive

Answer 10

a homozygous organism with two recessive alleles

Question 11

codominant

Answer 11

in some cases, two alleles both contribute to the phenotype

Question 12

multiple alleles

Answer 12

sometimes a gene has more than two allelic forms

- they occur in the human ABO blood grouping system

Question 13

What is a zygote?

Answer 13

The cell formed by the union of a sperm and an egg and the zygote develops into the embryo

Question 14

What is an autosome?

Answer 14

A chromosome not considered as a sex chromosome

Question 15

What is a sex chromosome?

Answer 15

A chromosome that involves the determination of the sex as well the development of sexual characteristics in an organism

Question 16

What is monohybrid inheritance

Answer 16

the inheritance of a single gene

Question 17

what is dihybrid inheritance

Answer 17

a cross between two different genes that differ in two observed traits

Question 18

An example of dihybrid inheritance

Answer 18

one of Gregor Mendel investigated the inheritance of two characters of a pea plant at the same time. These were:

• seed shape: where round shape is dominated to wrinkled shape
• seed colour: where yellow-coloured seeds are dominant to green-coloured ones

Question 19

When does co dominance occur?

Answer 19

when instead of one allele being dominant and the other recessive, both alleles are equally dominant. This means both alleles of a gene are expressed in the phenotype

Question 20

What are multiple alleles and give an example

Answer 20

A gene that may have more than two alleles so it has multiple. An example of this is the inheritance of human ABO blood groups
- There are 3 alleles associated with the gene ! (immunoglobulin gene) which lead to the presence of different antigens on the cell surface membrane of red blood cells:
- allele I^A which leads to the production of antigen A
- allele I^B which leads to the production of antigen B
- allele I^O which doesn’t lead to the production of either antigen
> alleles I^A and I^B are codominant, wheread allele I^O is recessive

Question 21

Sex inheritance in humans

Answer 21

In humans the sex chromosomes are X and Y this means:

• as females have two X chromosomes, all the gametes are the same in that they contain a single X chromosome
• males have one X chromosome and one Y chromosome, they produce two different types of gamete– half have an X chromosome and half have a Y chromosome

Question 22

what is haemophilia?

Answer 22

• blood clots slowly and there may be slow and persistent internal bleeding
• the condition is mostly confined to males because haemophillic females usually died with the onset of mestruation of puberty
• haemophilia is a recessive allele with an altered sequence of DNA nucleotide bases that codes for a faulty protein that doesn’t function for blood clotting

Question 23

inheriting haemophilia from mother

Answer 23

• as males only maintain their Y chromosome from their father ,it follows that their X chromosome from their mother
• As the defective allele that does not code for the clotting protein is linked to the X chromosome, males always inherit the disease
• if the mother doesn’t suffer from the disease, she may be heterozygous for the character (X^HX^h)
• as males pass the Y chromosome on to their sons, they can’t pass haemophilia to them, however they can pass the allele to their daughters via the X chromosome, who would then become carriers

Question 24

Autosomal Linkage

Answer 24

The name given to the situation where two or more genes are carried on the same autosomal linkage

Question 25

What are autosomes

Answer 25

the remaining 22 chromosomes other than sex chromosomes

Question 26

Mendel’s Law of Independent Assortment

Answer 26

Assuming there is no crossing over, all the linked genes remain together during meiosis and so pass into gametes and hence the offspring. They do not segregate in accordance with Mendel’s Law of Assortment

Question 27

When does Epistasis arise?

Answer 27

When the allele of one gene affects or masks the expression of another in the phenotype

Question 28

Other forms of Epistasis

Answer 28

where genes act in a sequence by determining the enzymes in a biochemical pathway