17- Inheritance

What is a genotype?

• The genetic constitution of an organism

• All the alleles that an organism has

What does the genotype determine, and what is an example?

• The limits within which the characteristics of an organism may vary

• For example, a person’s genotype may determine their maximum height

• Environment affects the actual maximum height the person may reach

• For example, a lack of calcium for the growth of bone will mean the person doesn’t reach their potential maximum height

What is the phenotype, and what is it a result of?

• The observable or biochemical characteristics of an organism

• It is the result of the interaction between the genotype and the environment

What is a gene?

• A length of DNA/ sequence of nucleotide bases

• That codes for a particular polypeptide

What are alleles?

Different versions of the same gene

How many alleles will one gene have?

Two or more

What is the locus of a gene?

The position of a gene on a particular DNA molecule

How many alleles can occur at the locus of one chromosome?

One

How can diploid organisms carry two alleles for the same gene, if only one allele can occur at on one chromosome?

• Chromosomes occur in pairs called homologous chromosomes

• Therefore there are two loci that each carry one allele of a gene

What is the organism described as if the alleles on both chromosomes are the same?

Homozygous

What is the organism described as if the alleles on both chromosomes are different?

Heterozygous

What is an organism with two dominant alleles described as?

Homozygous dominant

What is an organism with two recessive alleles described as?

Homozygous recessive

When will a recessive allele be present in the phenotype of a diploid organism?

When the organism is homozygous recessive only

The diagram shows two homologous chromosomes. What can the organism be described as?

Homozygous dominant

The diagram shows two homologous chromosomes. What can the organism be described as?

Heterozygous

The diagram shows two homologous chromosomes. What can the organism be described as?

Homozygous recessive

What are alleles which both contribute to the phenotype known as?

Codominant

What are the two potential consequences for an organism with two codominant alleles?

• The phenotype is either a blend of both alleles (e.g. pink petals from red codominant allele and white codominant allele)

• Both features are present (e.g. A and B antigens in blood group AB)

What does it mean if an organism has multiple alleles for a character?

The gene has more than two allelic forms

• Draw a monohybrid cross for when a parent with the genotype GG (green pod) is crossed with a parent with the genotype gg (yellow pod).

• Describe the genotypes, and phenotypes, of the offspring

• All offspring are heterozygous

• All offspring have green pods

• Here is a monohybrid cross for when a parent with the genotype GG (green pod) is crossed with a parent with the genotype gg (yellow pod).

• Draw a monohybrid cross for when two of the offspring are crossed.

• Describe the genotypes, and phenotypes, of the new offspring (F2 generation).

• One homozygous dominant, two heterozygous, one homozygous recessive

• Phenotypic ratio of 3:1- three have green pods, one has a yellow pod

Why is the phenotypic ratio in offspring almost never what’s expected?

Due to random fusion of gametes

How can you increase the chance of obtaining an actual phenotypic ratio close to the theoretical?

By increasing the sample size

What is dihybrid inheritance?

How two characters, determined by two different genes located on different chromosomes, are inherited

• Draw a dihybrid cross for when a plant with genotype RRGG (round and yellow seeds) is crossed with a plant with genotype rrgg (wrinkled and green seeds)

• Describe the genotypes and phenotypes of the offspring

• All offspring have the genotype RrGg

• All offspring produce round and yellow seeds

What is the first stage in drawing a monohybrid or dihybrid cross?

Determine the genotypes of the gametes of the parents, and place these along the top and down the side (for dihybrid, if both gametes are RG from one parent, you would have two RG gametes separately along the top)

• Here is a dihybrid cross for when a plant with genotype RRGG (round and yellow seeds) is crossed with a plant with genotype rrgg (wrinkled and green seeds)

• Draw a second cross for when two of the offspring are crossed

• Describe phenotypes of the new offspring

• Phenotypic ratio of 9:3:3:1 (9 round and yellow seeds, 3 round and green seeds, 3 wrinkled and yellow seeds, 1 wrinkled and green seed)

What does Mendel’s law of independent assortment state?

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

What is codominance?

Where both alleles are expressed in the phenotype if organism is heterozygous

How do you represent codominant and multiple alleles in a cross diagram?

• A capital letter that represents the gene

• Different letters for the alleles as superscripts

• For example, C^R and C^W for red and white coat colour

• Draw a monohybrid cross for when a cow with a red coat is crossed with a cow with a white coat

• The white and red alleles are codominant

• Describe the phenotype of the offspring

All offspring have both red hairs and white hairs

What is multiple alleles?

• If a gene has more than two alleles

• However, each individual can only have two of these at a time as there are only two homologous chromosomes

What is an example of a gene with multiple alleles?

• The inheritance of human ABO blood groups

• There are three alleles associated with the immunoglobin (I) gene

• I^A which leads to production of antigen A

• I^B which leads to production of antigen B

• I^O which doesn’t lead to the production of either antigen

• Draw a cross for when a parent with blood group AB (I^A and I^B) is crossed with a parent with blood group O (I^O and I^O)

• Describe the phenotypes of the offspring

• A and B are codominant alleles and O is recessive

50% of the offspring have blood group A

50% of the offspring have blood group B

What are the sex chromosomes in humans?

X and Y

What sex chromosomes do females have?

XX

What sex chromosomes do males have?

XY

What sex chromosomes will gametes produced by females contain?

All will have a single X chromosome

What sex chromosomes will gametes produced by males contain?

Half will have an X chromosome, half will have a Y chromosome

How do you describe a gene that is carried on the X or Y chromosome?

Sex-linked

Which is longer, the X or the Y chromosome?

The X chromosome

Why are males more likely to suffer from a recessive X-linked genetic disorder?

• Males are XY

• Only have to possess one recessive allele (as only have one X chromosome) in order to have the disorder

• Females are XX

• Must have two recessive alleles to have the disorder

What is an example of an X-linked genetic disorder?

Haemophilia

What is an X-linked genetic disorder?

A disorder caused by a defective gene on the X chromosome

How do you represent sex-linked alleles in a cross diagram?

• A capital letter as the sex chromosome

• Superscript capital or lowercase letters (for dominant and recessive alleles)

• For example, X^H and X^h

• Draw a cross diagram for when a carrier female for haemophilia is crossed with a normal male

• Haemophilia is a recessive X-linked genetic disorder

• Describe the phenotypes of the offspring produced

25% normal female

25% normal male

25% carrier female

25% male with haemophilia

If a female has a recessive X-linked disorder, what is the probability her son will also have the disease?

100% (males always inherit their X chromosome from their mother)

What are the non-sex chromosomes known as?

Autosomes

What is autosomal linkage?

Where two or more genes are carried on the same autosome

Why are autosomal-linked alleles inherited together?

• They are not separated during meiosis

• Assuming no crossing over

What is epistasis?

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

Explain epistasis in black, agouti, and albino mice.

• Gene A controls distribution of melanin- A leads to black bands and a leads to uniform black hairs

• Gene B controls coat colour- B leads to production of melanin, b leads to no melanin

• Therefore, if a mouse is bb (no melanin), gene A cannot be expressed and hairs cannot be uniform or banded (mouse is albino, and expression of A is affected by expression of B)

• Agouti mice are Bb or BB with Aa or AA

• Black mice are Bb or BB with aa

• Explain why this is epistasis

• Dominant allele for gene A codes for functional enzyme A, dominant allele for gene B codes for functional enzyme B

• If gene A is homozygous recessive, enzyme A will be non-functional and intermediate molecule will not be produced

• This means that, even if gene B is dominant, red pigment will not be produced as enzyme B has nothing to break down to red pigment

• Therefore epistasis because expression of gene A affects expression of gene B

What is the chi-squared test used for?

To test if the difference between the expected and the observed values in an investigation is statistically significant

What are the criteria for the chi-squared test?

• Sample size must be relatively large (over 20)

• Data must fall into discrete categories

• Only raw counts (not percentages, rates etc) can be used

What is the formula for the chi-squared test?

X² = ∑ ( (O - E)² / E )

In X² = ∑ ( (O - E)² / E ), what does X² represent?

Chi-squared

In X² = ∑ ( (O - E)² / E ), what does ∑ represent?

Sum of

In X² = ∑ ( (O - E)² / E ), what does O represent?

Observed numbers

In X² = ∑ ( (O - E)² / E ), what does E represent?

Expected numbers

What is the null hypothesis in a chi-squared test?

There is no significant difference between observed numbers and expected numbers

When do you reject the null hypothesis in a chi-squared test?

When the chi-squared value > critical value (at p= 0.05)

What is chi squared test often used for in genetics?

To test whether observed phenotypic ratios are statistically different from expected Mendelian ratios

What does it mean if you reject the null hypothesis in a chi-squared test?

• There is a significant difference between observed numbers and expected numbers

• That is not due to chance