3.7.1 inheritance

Gene definition:

Section of DNA located at a certain site on a DNA molecule (locus). The base sequence of a gene codes for amino acid sequence of a polypeptide.

Allele definition?

Different versions of a single gene

Phenotype definition?

Expression of the genetic constitution and its interaction with the environment.

What is the monohybrid and dihybrid cross method?

Monhybrid is a method used to predict the ratios of offspring considering a single gene, dihybrid crosses predict the ratio of offspring considering two genes.

Homozygous meaning?

At a specific locus, having a pair of identical alleles on a pair of homologous chromosomes.

Heterozygous meaning?

At a specific locus having a pair of non-identical alleles on a pair of homologous chromosomes.

Chromosome definition?

Linear DNA associated with histones

Homologous chromosomes definition?

Chromosomes with the same genes at the same loci.

Diploid and haploid cell definitions?

Diploid is a cell with 2 sets of chromosomes, haploid is a cell with one set of chromosomes.

On family trees, what is evidence that a condition is caused by a dominant allele?

Two parents who do have the condition have a child who does not, so both parents must be carriers of the recessive allele but due to the presence of the dominant allele they have the condition themselves.

On family trees, what is evidence that the condition is caused by a recessive allele?

Two parents who do not have the condition have a child who does have the condition. Therefore both parents must be carriers of the recessive allele but due to the dominant allele being present they do not have the condition.

What is codominance?

Where two alleles both contribute to the phenotype if present, and neither one is recessive so both are expressed.

How is codominance written when writing the alleles out?

Capital letter for the gene, then the allele in superscript.

What is the ratio of a monohybrid cross of two heterozygous parents?

3:1 (dominant to recessive)

What is the ratio of two heterozygous parents when the alleles are co-dominant?

1:2:1 (dominant to both phenotypes to recessive)

Why is the actual ratio of genetic crosses rarely the same as predicted results?

It is up to chance which ones fuse with which, and each fertilisation is independent of the one that came before it.

What are multiple alleles?

Where there are more than two alleles, of which only two may be present at the loci of an individual’s homologous chromosomes.

What gene codes for red blood cell antigens?

Immunoglobin gene (I)

What are the three alleles associated with the immunoglobin gene?

I^A which leads to the production of antigen A

I^B which leads to the production of antigen B

I^O which does not lead to the production of either antigen

What is the order of dominance for alleles for blood group?

Alleles A and B are codominant but O is recessive to both.

What blood type can each blood group have in blood transfusions?

A can have A and O

B can have B and O

AB can have AB, A, B and O

O can have O only

What antibody does each blood group have?

A has anti-B antibody, B has anti-A antibody, AB has no antibodies and O has both anti-A and anti-B antibodies

What are chromosomes that aren’t the sex chromosomes called?

Autosomes

Why are most sex linked diseases found on the X chromosome?

The X chromosome is much longer than the Y chromosome, so for most of the length of the X chromosome there is no equivalent homologous portion of the Y chromosome, so most sex linked genes are found on the X chromosome.

What does sex linkage mean?

Some characteristics being coded for by genes only found on sex chromosomes and not autosomes.

Why are characteristics controlled by recessive alleles that are sex linked more commonly found in men?

Because if on the X chromosome, males only have one allele present which will be expressed if it is the recessive allele, however in women they have two alleles of the gene, one of which may be dominant so the recessive allele is not expressed.

Why are sex linked genes in men passed down from female parents?

Because the Y chromosome will have come from the father, so the X chromosome must come from the mother.

Why can’t males be carriers of X-linked genes?

Because they only have one copy of the X chromosome, so if the allele is present it will always be expressed.

When may females express the X-linked gene in their phenotype if it is recessive?

Only if they are homologous for the recessive allele.

How is sex and characteristics represented in pedigree charts?

Male uses a square, female uses a circle, shading in either represents the characteristic present in the phenotype

What is dihybrid inheritance?

When two characteristics are inherited from two different genes which have different alleles and are on two different chromosomes.

What do genetic diagrams of dihybrid inheritance show?

The probability of inheriting certain combinations of the two different characteristics from the parents.

What two characteristics did Mendel investigate in his dihybrid inheritance crosses?

• Seed shape where round is dominant to wrinkled

• Seed colour where yellow is dominant to green

What genotype is always produced in the F1 generation from one homozygous dominant parent and one homozygous recessive for both traits in dihybrid inheritance?

All F1 offspring are heterozygous dominant for both characteristics.

What is the expected phenotypic ratio in the F2 generation when both parents are heterozygous dominant for both traits in dihybrid inheritance?

9:3:3:1

Why is the expected phenotypic ratio in dihybrid inheritance not always observed?

Due to linkage or epistasis

What is Mendel’s law of independent assortment?

Each member of alleles may combine randomly with either of another pair

What does it mean if chromosomes are linked?

Any two genes occurring on the same chromosome.

What is a linkage group?

Where sets of genes at different loci but on the same chromosome are more likely to be inherited together.

What is autosomal linkage?

Where two or more genes are carried on the same autosome.

What assumption does dihybrid inheritance rely on?

That the genes being investigated behave independently of each other during meoisis.

What happens to linked genes during meiosis assuming there is no crossing over?

All the linked genes remain together during meiosis and so pass into gametes and the offspring together, and do not segregate in accordance with the law of independent assortment, and so stay together during independent segregation in meiosis 1 (assuming there is no crossing over as there is less of a chance of this happening).

How are linked genes written?

The linked genes are written inside a bracket

What is the expected phenotypic ratio of an F2 dihybrid cross with autosomal linkage where both parents are heterozygous?

3:1

Why is the hypothetical phenotypic ratio for autosomal linkage not usually the same as observed ratio?

There is a small percentage of crossing over producing some recombinant genotypes

What are the rules for when to use chi squared test?

• Sample size must be quite larger - over 20

• Data must fall into discrete categories

• Only raw data counts

• Used to compared experimental results with theoretical ones

What does it mean in inheritance if the X2 value is smaller than the critical value of 0.05?

That there is no significant difference between the observed and expected phenotypic ratios, so the Null hypothesis can be accepted because there is a less than 5% chance that genes did not assort randomly.

What is epistasis?

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

What is an example of epistasis in terms of enzyme production?

• Where genes act in sequence by determining the enzymes in a biochemical pathway, so if the first gene is recessive and doesn’t produce the enzyme for the intermediate molecule then even if the second gene is dominant and produces the enzyme the phenotype will not be expressed

In Hardy Weinberg why might actual frequencies differ from expected frequencies?

Emigration or immigration

No random mating

High mutation rate

Selection for a certain phenotype

what is evidence from a genetic diagram that X linked dominance has occurred?

100% incidence of affected daughters from an affected father suggests X linked dominance.

what is evidence from the genetic diagram that it is X linked recessive?

100% incidence from affected sons from an affected mother suggests X linked recessive