8.2 - Transfer of genetic information

What is the definition of genotype & phenotype?

• Genotype: the combination of alleles an organism possesses

• Phenotype: the observable characteristics of an individual, determined by both genotype & environment

What is an allele?

Different forms of a particular gene, found at the same locus (position) on a chromosome

What is the definition of dominance & recessive?

• Dominance: a trait which is present if an individual has at least one copy of the allele

• Recessive: a trait which is only present when an individual has two copies of the allele & can be masked by a dominant allele

What is the definition of codominance & multiple alleles?

• Codominance: two dominant alleles for a gene in a heterozygous organism equally contribute to the phenotype

• Multiple alleles: a gene with more than two alleles which can code for a characteristic

What is the definition of homozygote & heterozygote?

• Homozygote: an organism which has two of the same versions of a gene (i.e. both alleles are dominant or recessive)

• Heterozygote: an organism which has two different versions of the same gene (i.e. one allele is dominant, the other is recessive)

Draw a genetic cross diagram for two heterozygous parents with blue eyes & determine the offspring’s ratio:

• (B) = blue allele

• (b) = brown allele

What is a pedigree diagram?

Can be used to trace the pattern of inheritance of a specific trait through generations of a family. They can provide information, such as:

• whether a trait is caused by a dominant or recessive allele

• whether a trait is more likely to be inherited by males or females

• the genotype of individuals in the family

• the probability that an individual in the family will inherit a trait

The pedigree diagram traces the inheritance of albinism through several generations. (A) = normal allele; (a) = albinism allele

Using the pedigree chart, deduce & explain the following:

1. the type of allele that causes albinism

2. the genotype of individuals 9 & 7

3. the possible genotypes of 10 & 11

Question 1:

• Albinism is caused by a recessive allele

• Person number 9 is an affected individual, despite parent 6 & 7 being unaffected

• 6 & 7 must both be carriers of the recessive allele & 9 has inherited one recessive allele from each parent

Question 2:

• The genotype of person 9 must be homozygous recessive (aa) & the genotype of 7 must be heterozygous (Aa)

• Person 9 is a affected individual with albinism; as this is determined by the recessive allele, they must have two copies of the albinism allele

• Person 7 must be heterozygous, as he does not have albinism but has passed on the recessive allele to person 9

Question 3:

• The possible genotypes of 10 & 11 are heterozygous (Aa) or homozygous dominant (AA)

• They are both unaffected individuals, so must possess at least one dominant allele (A), however, it is possible they each might have inherited a recessive alle (a) from one parent (both parents must have a copy of the recessive allele for person 9 to have albinism)

How are non-interacting, unlinked genes inherited?

Can be monohybrid or dihybrid:

• monohybrid = a phenotypic characteristic is controlled by a single gene

• dihybrid = two phenotypic characteristics are controlled by two different genes present on two different chromosomes

What is meant by autosomal linkage?

Two or more genes are located on the same (non-sex) chromosome:

• in this case, only one homologous pair is needed for all four alleles to be present

• for genes that aren’t linked, two homologous pairs are needed

What is an example of autosomal linkage?

Drosophila (fruit flies):

• colour & wing length are autosomally linked, meaning they are inherited pairs

Explain why are autosomally linked genes are inherited in pairs, referring to meiosis

• Genes on the same chromosome, particularly those closest together, are unlikely to undergo recombination (i.e. independent assortment & crossing over) during meiosis

• Therefore, they are inherited as if they were the same gene

What is meant by sex-linkage & give an example?

An allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual (e.g. haemophilia)

Why are males more likely to express a recessive sex-linked allele than females?

• Most sex-linked alleles are located on the X chromosome

• Therefore, males only get one copy of the allele (i.e. XY), so will express this characteristic even if it’s recessive

• Since females get two alleles (i.e. XX), this is less likely

What is the chi-squared test?

A statistical test to find out whether the difference between observed & expected data is due to chance or a real effect

What are the key ideas for the chi-squared test?

• Observed (O): what you counted for each phenotype/class

• Expected (E): what you should get from the genetic ratio, scaled to your total

• X² compares O & E across categories → smaller X² usually means differences could be due to chance

• Degrees of freedom (df) = number of categories - 1

• Decision rule at p= 0.05

  • if X² is less than critical value → accept

  • if X² is greater than or equal to the critical value → reject

What is the formula for the chi-squared test?

(apply to each category, then sum):

What is the step-by-step method for chi-squared?

1. State the genetic hypothesis & its expected ratio

2. Find expected counts: multiply each ratio fraction by the total

3. Compute (O - E)² / E for each category & add them to get total X²

4. Work out degrees of freedom: df = number of categories - 1

5. Use the critical values table (usually p = 0.05) to decide accept/reject

Answer this chi-squared question:

Monohybrid 3:1 expected. Observed in 200: 140 dominant, 60 recessive. Test at p = 0.05

Answer this chi-squared question:

Dihybrid 9:3:3:1 expected. Observed in 256: 150, 60, 30, 16. Test fit at p = 0.05

Answer this chi-squared question:

1:2:1 genotype ratio expected in an F2 (e.g. AA:Aa:aa). Observed in 160: 30AA, 100Aa, 30aa. Test at p = 0.05