Inheritance

Question 1

Define gene

Answer 1

A section of DNA that codes for a protein.

(Long ver: They control the characteristics of an organism by coding for the protein which results in that characteristic)

Question 2

Define genotype.

Answer 2

The genetic constitution of an organism (what genes it has)

Question 3

Define phenotype.

Answer 3

The expression of its genetic constitution and its reaction to the environment (i.e. its physical appearance)

Question 4

Define allele.

Answer 4

Different forms of a gene.

Question 5

Define homozygous.

Answer 5

When an organism has two copies of a gene with the same allele (e.g. BB, dd, FF)

Question 6

Define heterozygous.

Answer 6

When an organisms has two copies of a gene with different alleles (e.g. Bb, Dd, Ff)

Question 7

Define dominant allele.

Answer 7

An allele that is always expressed, represented by uppercase letters.

Question 8

Define recessive allele.

Answer 8

An allele that is only expressed if genotype is homozygous, represented by lowercase letters.

Question 9

What would ‘RR’ be classified as?

Answer 9

Homozygous dominant

Question 10

What would ‘Rr’ be classified as?

Answer 10

Heterozygous

Question 11

What would ‘rr’ be classified as?

Answer 11

Homozygous recessive

Question 12

Are adults haploid or diploid?

Answer 12

Diploid

Question 13

Are gametes (egg & sperm) haploid or diploid?

Answer 13

Haploid

Question 14

What is a genetic cross?

Answer 14

Its a diagram that shows the way in which alleles are passed from one generation to the next. The alleles are represented as letters.
P is used to indicate a parental generation
F1 refers to the offspring to the parents
F2 refers to the offspring of the F1 gen and so on

Question 15

What is monohybrid inheritance?

Answer 15

It is the inheritance of a single gene which determines a single characteristic.

Question 16

How do you make and use Punnet squares?

Answer 16

1. Determine parental genotypes
2. Split the genotype into its alleles and make a grid
3. Work out the possible combinations
4. Show ratio of the genotype and phenotypes e.g.
   RR:Rr:rr
   1:2:1
   Red(dominant):yellow(recessive)
   3:1

Question 17

What is dihybrid inheritance?

Answer 17

This is the inheritance of a two different genes which determines a two different characteristic. The principles are the same as monohybrid inheritance. The example of a cross both parents are heterozygous.

E.g. Parents genotypes: RrLl & RrLl
Produce gametes: RL,Rl,rL,rl & RL,Rl,rL,rl

Question 18

What is co-dominance?

Answer 18

Some alleles do not show a standard dominant/recessive relationship and if they occur together in a heterozygous individual, an intermediate phenotype is produced (the actual reason why is complex and varies from case to case). We write co-dominance with gene letter is used and superscript letters are used for the allele.
E.g. Red flower allele + white flower allele = pink flower

Question 19

What is multiple allele inheritance?

Answer 19

For many genes, there are several alleles in the population. These may show a dominance hierarchy.

A common example of multiple alleles is blood types. Blood grouping not only is an example of multiple allele but also co-dominance. IO is recessive in relation to IA and IB.. Allele IA and IB display co-dominance.

Question 20

What is autosomal linkage?

Answer 20

In humans there are 23 pairs of chromosomes. 22 of these pairs are known as homologues pairs, these are known as autosomes. The last pair are sex chromosome which determine the sex of the individual.
Each pair of homologues chromosomes contains one maternal autosome and one paternal autosome. They carry the same genes, but they may be different alleles.

The each autosome (chromosome) has the ability to carry different genes; all the genes on one autosome are classed as linked and is known as autosomal linkage. They will stay together during independent segregation/random assortment, therefore they will be passed on to the offspring all together, the only way they could be separated is crossing over.

Question 21

What is sex linkage?

Answer 21

In humans one of the twenty three pairs of chromosomes are referred to as the sex chromosomes. They carry the genes which determine sex.
There are two different types of sex chromosomes, the X and the Y chromosome.
The Y chromosome only carries sex genes.
The X chromosome carries both sex genes and genes controlling other body characteristics (autosomal genes).

If an individual has two X chromosomes (homogametic), they will be female.
If an individual has one X and one Y chromosome (heterogametic), they will be male.

The X chromosome carries normal body genes, and these are linked to the genes which determine sex, so these body genes are referred to as being sex-linked genes.
Consider sex-linked genes found on the X chromosome (labelled A on the diagram). In the male, alleles a have no corresponding allele on the Y chromosome, so these recessive alleles will always be expressed in the phenotype.
Females therefore have the ability demonstrate a heterozygous genotype, whereas males can only display a dominant or recessive genotype and therefore are more likely to suffer from recessive characteristic compare to females. Females can be a carries of recessive diseases.

Question 22

What is epistasis?

Answer 22

The idea that one gene may affect the expression of another gene within the phenotype, specifically the ability to mask the expression of genes.

E.G.
If an individual is homozygous recessive for gene A (melanin produced or not) they would not produce melanin and therefore be classified as albino, in mouse coat colour the phenotype appears as white.

The genes control an enzyme pathway, which demonstrates the need for gene A (melanin produced or not) to be expressed prior the expression of gene B.

Question 23

What are pedigrees?

Answer 23

Genetic inheritance is often shown in the form of a pedigree (family tree).

Dominant Traits:
To prove the disease is dominant.
Evidence: (always use numbers/ names as evidence)
-If a child does not have a disease and parents do
-Parents are heterozygous

Recessive Trait:
To prove the disease is recessive.
Evidence: (always use numbers/ names as evidence)
-If a child has a disease and parents do not
- Parents are heterozygous

Sex linked traits:
Evidence: (always use numbers/ names as evidence)
* Daughter must receive fathers X chromosome
* If daughter has the (recessive) disease then father must have it if it’s sex linked.
* Son must receive fathers Y chromosome

Question 24

The ABO blood group system in humans is an example of multiple allele inheritance. Using this system, human blood can be classified into four possible blood groups: A, B, AB and O. The blood group of a particular individual is determined by a single gene pair.
With reference to the inheritance of blood group in the ABO system, explain codominance. (2)

Answer 24

idea of both alleles (in heterozygote) contributing (equally) to expression (in phenotype) ;

{ I A / allele for A} is codominant with { IB / allele for B} / IA and I B are both dominant over IO;

Question 25

The ABO blood group system in humans is an example of multiple allele inheritance. Using this system, human blood can be classified into four possible blood groups: A, B, AB and O. The blood group of a particular individual is determined by a single gene pair.
With reference to the inheritance of blood group in the ABO system, explain mutiple allele inheritance. (2)

Answer 25

idea of more than two alleles available at a locus ;

idea of three alleles in blood grouping / reference to IA, IB or Io being available ;