Biology - Chapter 17: Inherited Change

Genotype

The genetic constitution of an organism

Phenotype

The expression of the genotype and its interaction with the environment

Homozygous

When the allele on each chromosome of a pair of homologous chromosomes is the same

Heterozygous

When the allele on each chromosome of a pair of homologous chromosomes is different

Dominant

An allele that is always expressed

Recessive

An allele that is only expressed if two copies of it are present

Homozygous dominant

Homozygous organism with two dominant alleles

Homozygous recessive

Homozygous organism with two recessive alleles

Codominant alleles

Two alleles that are both expressed in the phenotype if both are present

How do you construct a Punnet square? (4)

1. Write out phenotype and genotype of parents
2. Write out gametes of parents
3. Draw in table - identify which gametes are male or female
4. Identify phenotypes of offspring and calculate phenotypic ratios

Monohybrid inheritance

The inheritance of a single gene

Pure-breeding

When homozygous individuals are bred together to consistently produce offspring with the same phenotype

F1 generation meaning

First filial generation

First filial generation

The first generation of offspring

The law of segregation

In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete.

What happens to the results as the sample size increases?

The larger the sample, the more likely the actual results come near to matching the theoretical ones

Dihybrid inheritance

The inheritance of two characteristics that are determined by two different genes located on two different chromosomes

Law of independent assortment

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

Describe the multiple alleles situation

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

Expected phenotypic ratio for monohybrid heterozygous cross?

3:1

Expected phenotypic ratio for dihybrid heterozygous cross?

9:3:3:1

2 sex chromosomes

X and Y chromosomes

Compare the X and Y chromosomes

Y chromosome is smaller and shaped differently

Sex chromosomes in females

XX

Sex chromosomes in males

XY

Sex-linked

A gene carried on the sex chromosomes

Why do characteristics controlled by recessive alleles on the X chromosome appear more frequently in males?

Males have one allele, while females need two recessive alleles (must be homozygous recessive)

How do you express sex-linked genes?

Chromosome^{characteristic} e.g. X^N/Xⁿ

Carrier

When someone is heterozygous for a characteristic so carry the allele without it being expressed in their phenotype

Pedigree chart

A way of tracing the inheritance of sex-linked characters

Linked genes

Any 2 genes that occur on the same chromosome

Linkage group

All the genes on the same chromosome

Autosomes

All the chromosomes that are not sex chromosomes

Autosomal linkage

The situation where two or more genes are carried on the same autosome

How many possible combinations of the alleles in gametes are there for linked genes (A and B) and what is the assumption made?

AB and ab - assume there is no crossing over

When does epistasis arise?

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

What do you test with the chi-squared test?

The null hypothesis - testing whether there is any deviation between the observed and expected numbers in an investigation and if it is significant or not

3 criteria to carry out chi-squared test

1. Sample size must be relatively large i.e. over 20
2. Data must fall into discrete categories
3. Only raw counts can be used

Chi-squared formula

sum of(observed numbers - expected numbers)^2 / expected numbers

How do you carry out chi-squared test?

Use formula, then read value of distribution table based on calculated degrees of freedom

In observed ratio of cross for dihybrid inheritance, why might the results for the homozygous dominant/recessive offspring be close in value and no heterozygous offspring are produced? (3)

The two genes are linked (i.e. autosomal linkage) and no crossing over occurs, so only homozygous dominant/recessive gametes are produced

Expected phenotypic ratio for dihybrid cross with codominant characteristics AaBB and AABb?

1:1:1:1

Explain results of observed ratio of cross for dihybrid inheritance where homozygous dominant/recessive offspring are close in value and there are few heterozygous offspring. (4)

1. The two genes are linked (e.g. GN, gn)

2. So GN and gn gametes are mostly produced

3. Crossing over occurs, so few Gn and gN gametes are produced

4. So there are fewer individuals with heterozygous genotypes (e.g. Ggnn and ggNn)

There are two characteristics: 1 dominant and 1 recessive. If both parents have the dominant characteristic and one of the offspring has the recessive characteristic, what are the parent’s genotypes and why? (3)

They’re heterozygous - both parents have dominant characteristic but offspring have recessive characteristic, so PARENTS MUST BE CARRIERS of recessive allele

Why are the observed phenotypic ratios of monohybrid inheritance / general genetic crosses not the same as the expected ratios? (5)

1. Small sample size

2. Random fertilisation of gametes

3. Lethal alleles / selection advantage

Generally:

4. Epistasis

5. Linked genes

When do you use the chi-squared test?

When you have categorical data

How do you calculate the degrees of freedom for a chi-squared test?

Number of classes - 1

What does it mean if the result calculated in a chi-squared test is less than the critical value?

Difference is not significant - accept null hypothesis

What does it mean if the result calculated in a chi-squared test is more than the critical value?

Difference is significant - reject null hypothesis

What is the chi-squared test used for?

To compare experimental results with theoretical ones