3.7.1 inheritance

Define genotype

The genetic constitution of an organism

Define phenotype

The appearance of a characteristic due to the expression of the genotype + its interaction with the environment

What are alleles?

Different forms of the same gene

What are dominant + recessive alleles?

Dominant → always expressed in the phenotype

Recessive → only expressed when genotype is homozygous recessive

What is the locus?

The position of a gene on a chromosome

In a diploid organism, the alleles at a specific locus may be either…

homozygous or heterozygous

What does homozygous + heterozygous mean?

Homo - refers to both alleles being the same

Hetero - refers to both alleles differing

What is monohybrid inheritance?

Refers to the inheritance of one particular characteristic

What did Mendel do?

• Studied the inheritance of height in pea plants → crossed pure breeding (homozygous) tall + dwarf plants

• All offspring were tall → then inbred this first gen which produced occurred in the ratio 3 tall : 1 dwarf

What are test crosses used for?

To determine the genotype of an organism with a dominant phenotype → always involves crossing the organism with the homozygous recessive

*remember probabilities are multiplied

What is codominance?

Where both alleles are expressed in the phenotype → neither allele is dominant with both contributing to the phenotype of the characteristic

What are multiple alleles?

Where there are more than 2 alleles of each particular gene → but only 2 alleles can be present in a genotype

What is an example of codominance and multiple alleles?

ABO blood group

What are the 4 phenotypes in the ABO blood group?

A, B, AB and O

What are autosomes?

The 22 pairs of non-sex chromosomes

What is the one pair of chromosomes?

XX (females) or XY (males) → probability for either is 50%

What does it mean if a gene is sex linked?

Where the gene is carried on only one type of sex chromosome (usually the X)

Give some examples of sex linked conditions

• Colour blindness

• Haemophilia → inability to clot blood

What causes colour blindness?

• Caused by a recessive allele carried on the X chromosom

• Allele B for normal colour vision is dominant while b for colour blindness is recessive

• Y chromosome doesn’t carry this allele

What is haemophilia?

Recessive condition in which blood doesn’t clot + suffered bleed to death if not treated → carried on the X chromosome

When is a woman a carrier of haemophilia?

She has both dominant + recessive allele → doesn’t express the condition

genotype - XHXh

Draw the punnet square for the offspring of a carrier female + normal male for haemophilia

If a gene is sex linked, it can be assumed to be what linked?

X linked

Why can’t a man pass a sex linked trait to his son?

Males have a Y chromosome which could only come from their fathers so their X chromosome must have come from their mothers → any X-linked genes are guaranteed to be passed from mother to son

Why do recessive sex linked traits to occur most often in males?

There is no other allele on the Y chromosome so recessive allele is always expressed → this doesn’t mean females can’t get the disease - just less likely as they have to inherit 2 recessive alleles

What is a useful + common way to show / trace the inheritance of genes?

With pedigree charts

What is the evidence + explanation for these?

What is dihybrid inheritance?

Refers to inheritance of 2 different characteristics controlled by 2 different genes

e.g in pea plants one gene controls the shape of seeds produced + a different gene controls the colour of the seed

What is referred to as Mendel’s Dihybrid ratio + what is it dependent on?

9 : 3 : 3 : 1

dependent on the fertilisation of gametes between the 2 plants being random

The observed + expected ratios may not be similar for what reasons?

• Epistasis

• Linked genes

• Fusion of gametes at fertilisation is random

• Small sample size → if there’s a low number of offspring then the sampling error is greater + less likely that observed ratio will be the same as the expected ratio

What is the chi-squared test used to do?

Compare the ‘goodness of fit’ of observed phenotypic ratios with expected ratios → used to determine if there’s a statistical difference between the expected + observed ratios from the results of a genetic cross

What are autosomal linked genes?

Genes that are present on the same chromosome at different loci

What are the implications of these linked genes tending to be inherited together?

• No independent assortment → fewer genetic recombinations of their alleles are possible in the gametes

• Which leads to reduced variety of gametes produced → reduces variety of offspring that can be produced

What does the expected ratio for linked genes assume?

That no crossing over (recombination) has occurred between the pair of homologous chromosomes carrying the linked genes → crossing over can separate the alleles of linked genes leading to new allele combinations + recombinants

Why will the recombinant gametes occur in much lower numbers than the parental gametes?

Crossing over is rare

What is epistasis?

When 2 or more genes interact to contribute to a phenotype → allele of one gene affects / masks the expression of another gene at another locus

When does epistasis often occur?

In metabolic pathways controlled by enzymes coded for by different genes → here, each enzyme is dependent on the previous enzyme for its substrate + if any of the enzymes are non-functional, pathway comes to a halt