Genetics

Question 1

Gene

Answer 1

A heritable factor that controls a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus)

Question 2

Allele

Answer 2

One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same locus as other alleles of the gene

Question 3

Genome

Answer 3

The whole of the genetic information of an organism

Question 4

Gene mutation

Answer 4

A change in the nucleotide sequence of a section of DNA coding for a particular feature

Question 5

Explain the consequence of a base substitution mutation in relation to the process of transcription and translation using the example of sickle cell anaemia

Answer 5

A base substitution mutation is the change of a single base in a sequence of DNA, resulting in a change to a single mRNA codon during transcription

Question 6

sickle cell anaemia

Answer 6

he 6th codon for the beta chain of haemoglobin is changed from GAG to GTG (on the non-coding strand)

This causes a change in the mRNA codon (GAG to GUG), resulting in a single amino acid change of glutamic acid to valine (Glu to Val)

• DNA: GAG to GTG (non-coding strand) • mRNA: GAG to GUG • Amino Acid: Glu to Val

The amino acid change alters the structure of haemoglobin, causing it to form fibrous, insoluble strands

This causes the red blood cell to adopt a sickle shape

Question 7

Consequences of Sickle Cell Anaemia

Answer 7

The insoluble haemoglobin cannot effectively carry oxygen, causing individual to feel constantly tired

The sickle cells may accumulate in the capillaries and form clots, blocking blood supply to vital organs and causing a myriad of health problems

Also causes anaemia (low RBC count), as the sickle cells are destroyed more rapidly than normal red blood cells

Sickle cell anaemia occurs in individuals who have two copies of the codominant ‘sickle cell’ allele (i.e. homozygotes)

Heterozygous individuals have increased resistance to malaria due to the presence of a single ‘sickle cell’ allele (heterozygous advantage)

Question 8

Codominant alleles:

Answer 8

pairs of alleles that both affect the phenotype when present in a heterozygote. (The terms incomplete and partial dominance are no longer used.)

Question 9

Locus

Answer 9

he particular position on homologous chromosomes of a gene.

Question 10

Carrier

Answer 10

an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

Question 11

Test cross

Answer 11

testing a suspected heterozygote by crossing it with a known homozygous recessive. (The term backcross is no longer used.)

Question 12

4.3.3 State that some genes have more than two alleles (multiple alleles)

Answer 12

Share codominance (be expressed equally in the phenotype) 
Share incomplete dominance (neither is fully expressed in the phenotype, resulting in blending) 
Demonstrate a dominance order (e.g. allele A > allele B > allele C)

Question 13

Describe ABO blood groups as an example of codominance and multiple alleles

Answer 13

I stands for immunoglobulin (antigenic protein on blood cells)
A and B stand for the codominant variants

The ABO gene has three alleles: IA, IB and i

IA and IB are codominant, wherease i is recessive (no antigenic protein is produced)
Codominance means that both IA and IB alleles will be expressed within a given phenotype

Question 14

Explain how sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans

Answer 14

The 23rd pair of chromosomes are heterosomes (or sex chromosomes) and determine gender

Females are XX - they possess two X chromosomes
Males are XY - they posses one X chromosome and a much shorter Y chromosome

The Y chromosome contains the genes for developing male sex characteristic - hence the father is always responsible for determining gender

If the male sperm contains the X chromosome the growing embryo will develop into a girl
If the male sperm contains a Y chromosome the growing embryo will develop into a boy
In all cases the female egg will contain an X chromosome (as the mother is XX)

Because the X and Y chromosomes are of a different size, they cannot undergo crossing over / recombination during meiosis
This ensures that the gene responsible for gender always remains on the Y chromosome, meaning that there is always ~ 50% chance of a boy or girl

Question 15

4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome

Answer 15

The Y chromosome is much shorter than the X chromosome and contains only a few genes

Includes the SRY sex-determination gene and a few others (e.g. hairy ears gene)

The X chromosome is much longer and contains several genes not present on the Y chromosome

Includes the genes for haemophilia and red-green colour blindness

In human females, only one of the X chromosomes remains active throughout life

The other is packaged as heterochromatin to form a condensed Barr body
This inactivation is random and individual to each cell, so heterozygous women will be a mosaic - expressing both alleles via different cells

Question 16

4.3.7 Define sex linkage

Answer 16

Sex linkage refers to when a gene controlling a characteristic is found on a sex chromosome (and so we associate the trait with a predominant gender)

Sex-linked conditions are usually X-linked, as very few genes exist on the shorter Y chromosome

Question 17

4.3.8 Describe the inheritance of colour blindness and haemophilia as examples of sex linkage

Answer 17

Colour blindness and haemophilia are both examples of X-linked recessive conditions
The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome)
As males only have one allele for this gene they cannot be a carrier for the condition
This means they have a higher frequency of being recessive and expressing the trait
Males will always inherit an X-linked recessive condition from their mother
Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents

Question 18

State that a human female can be homozygous or heterozygous with respect to sex-linked genes

Answer 18

As human females have two X chromosomes (and therefore two alleles for any given X-linked gene), they can be either homozygous or heterozygous

Males only have one X chromosome (and therefore only one allele) and are hemizygous

Question 19

4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles

Answer 19

An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier
Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves
Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers
Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater

Question 20

Autosomes

Answer 20

Pairs of chromosomes that are identical in appearance (e.g. same size, same gene loci, etc.) and are not involved in sex determination

Question 21

Sex chromosomes

Answer 21

Pairs of chromosomes involved in sex determination and are not identical in appearance (e.g. X and Y chromosome in humans)

Question 22

10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in the exchange of alleles

Answer 22

During crossing over in prophase I, non-sister chromatids of a homologous pair may break and reform at points of attachment called chiasmata
As these chromatids break at the same point, any gene loci below the point of the break will be exchanged as a result of recombination
This means that maternal and paternal alleles may be exchanged between the maternal and paternal chromosomes, creating new gene combinations
The further apart two gene loci are on a chromosome, the more likely they are to be exchanged

Question 23

10.2.4 Define linkage group

Answer 23

A linkage group is a group of genes whose loci are on the same chromosome and therefore do not follow the law of independent assortment
Linked genes will tend to be inherited together - the only way to separate them is through recombination (via crossing over during synapsis)

Question 24

10.2.5 Explain an example of a cross between two linked genes

Answer 24

When two genes are linked, they do not follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents
Instead the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together
This means that offspring will tend to produce the parental phenotypes
Recombinant phenotypes will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low numbers (if at all)
An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant
Example of a Cross between Two Linked Genes

Question 25

10.3.1 Define polygenic inheritance

Answer 25

Polygenic inheritance refers to a single characteristic that is controlled by more than two genes (also called multifactorial inheritance)
Polygenic inheritance patterns normally follow a normal (bell-shaped) distribution curve - it shows continuous variation
By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous

Question 26

Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour

Answer 26

Human Skin Colour

The colour of human skin is determined by the amount of dark pigment (melanin) it contains
At least four (possibly more) genes are involved in melanin production; for each gene one allele codes for melanin production, the other does not
The combination of melanin producing alleles determines the degree of pigmentation, leading to continuous variation
TED Talks: Inheritance of Human Skin Colour

Grain Colour in Wheat

Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain
Three genes control the colour and each gene has two alleles (one coding for red pigment, the other coding for no pigment)
The most frequent combinations have an equal number of ‘pigment producing’ and ‘no pigment’ alleles, whereas combinations of one extreme or the other are relatively rare
The overal pattern of inheritance shows continuous variation