Chapter 17 – Genetics Crosses:

2.5 Genetics Objectives

2.5.6 Genetic

Inheritance

1. Define a gamete and discuss their formation

2. Define fertilisation

3. Discuss sex determination

4. Define allele and differentiate between the

terms homozygous and heterozygous

5. Differentiate between genotype and phenotype

6. Differentiate between dominant and recessive

7. Explain incomplete dominance

8. Be able to complete monohybrid crosses and

state the genotypes and phenotypes of parents

and offspring

9. Understand the 3:1 ratio for heterozygous

crosses

2.5.10.H Origin of the

Science of genetics

10. Discuss the work of Gregor Mendel

2.5.11 H Law of

segregation

11. State the Law of Segregation

12. Discuss the chromosomal basis for this law

13. Complete monohybrid crosses

2.5.12 H Law of

independent assortment

14. State Mendel’s Law of Independent Assortment

15. Discuss the chromosomal basis for this law

2.5.13.H Dihybrid cross 16. Complete dihybrid crosses

17. Know the expected ratios from Mendel’s Laws

18. Define linkage and identify linked genes

19. Define sex linkage and give examples of sex

linked characteristics

20. Discuss non-nuclear inheritance

• Somatic cells consist of all of the body cells except the sex cells, they are diploid

(2n) and contain 46 chromosomes.

• Gametes are sex cells. They are haploid cells (n) which are capable of fusion.

• In humans gametes are the egg and sperm. Their function is to pass genes from one

generation to another during sexual reproduction.

- Gametes are made during meiosis (chromosome number is halved allowing

fertilisation to occur).

Your genes work with your environment to determine your traits/characteristics.

Fertilisation is the union of two gametes to form a single cell called a zygote

Genotype is the genetic make-up of an organism.

Phenotype is the physical make-up or appearance of an organism.

Phenotype = Genotype + Environment

Diploid cells contain 2 copies of each chromosome, this means 2 copies of every gene.

- The locus of a gene is its position on a chromosome.

If a cell has two of the same alleles they are said to be homozygous, if they have two

different alleles they are heterozygous.

A Dominant allele determines the phenotype. Dominant alleles prevent recessive alleles

from being expressed.

If an allele is recessive it is prevented from being expressed by the dominant form.

Possible genotypes include: - Heterozygous/Incompletely Dominant (Bb)

- Homozygous Dominant (BB)

- Homozygous Recessive (bb)

Genetics crosses involve following the genes of both parents and how they combine to

produce offspring.

- A Punnett Square is used to show the ratio of the offspring of a genetic cross.

- Progeny is the offspring of a cross.

- F1 is the first filial generation, i.e. the first generation of offspring

- F2 is the second filial generation, i.e. if you cross an F1 with another

Alleles are different forms of the same gene.

Homozygous genes have two identical alleles for the same trait.

Heterozygous genes have two different alleles for the same trait.

Codominance/Incomplete dominance means neither allele is dominant or recessive

with respect to the other. Both alleles are equally expressed in the heterozygous

genotype to produce an intermediate phenotype.

Monohybrid Crosses- One characteristic/trait

Homozygous Cross

In pea plants, green pods (G) is dominant over yellow pods (g). Give the genotypes and

phenotypes for the offspring of a cross involving two homozygous pea plants whose

genotypes are (GG) and (gg).

Heterozygous Cross

In Humans, the gene for brown eyes (B) is dominant to that for blue eyes (b). Give the

genotypes and phenotypes for the offspring of a cross involving two heterozygous

parents.

Incomplete Dominance Cross

A Pedigree is a diagram showing the genetic history of a group of related individuals.

Every somatic cell in the body contains 46 chromosomes.

- 44 are autosomes - 2 are sex chromosomes

- Autosomes control all non-gender related features and traits.

- The sex chromosomes are X and Y. They control gender related traits.

- In humans females are XX and males are XY. Y is half the size of X.

- It is the male who determines the sex of the child.

- The ratio of male to female births should be equal, 50/50, 1:1

The cross to show this ratio is:

Gregor Mendel

• Gregor Mendel is known as ‘the father of genetics’.

• Mendel carried out experiments on pea plants and studied the inheritance of 7

characteristics in them, including flower colour, stem height, and seed shape.

- His experiments involved transferring pollen between flowers by hand, and

growing the seeds in labelled pots to see which trait (phenotype) was passed on.

- His success was down to two key features.

• He only studied features that displayed two forms: yellow or green, tall or small,

round or wrinkled.

• He counted the number of plants with each type of trait and calculated

mathematical ratios around how traits were inherited: 1:1 or 3:1 etc.

Mendel’s Law’s

Chromosomal Basis of 1st Law: In diploid cells chromosomes occur in homologous pairs.

Alleles are located at the same locus on each chromosome in the pair. During meiosis,

homologous chromosomes separate into different haploid cells. This means the alleles

separate.

Chromosomal Basis of 2nd Law: During meiosis (gamete formation) each homologous

chromosome is equally likely to combine with any other homologous chromosome from a

pair.

Dihybrid Crosses- Two characteristics/traits

Both parents homozygous for both traits

In Pea-plants, tall plant (T) is dominant over small plant (t). In addition green pod (G) is

dominant over yellow pod (g). A tall plant with green pods (homozygous for both traits)

is crossed with a small plant with yellow pods. Show the genotypes and phenotypes of

the F1 progeny of this cross.

1st Law of Segregation Inherited characteristics are controlled by pairs of alleles.

These alleles segregate from each other at gamete formation, with only one member

of the pair being found in each gamete.

2nd Law of Independent Assortment When gametes are formed, either pair of

alleles, is equally likely to combine with either of another pair of alleles.

One parent heterozygous, one parent homozygous recessive for both traits

In Guinea-pigs black coat (B) is dominant to white coat (b). Also short hair (S) is

dominant to long hair (s). Show the expected genotypes and phenotypes of the progeny

from a cross involving a black-coated, short-haired guinea-pig (heterozygous for both

traits) and a white-coated, long-haired animal.

A selfed (F2) cross

A homozygous purple-flowered, short-stemmed plant is crossed with a red-flowered,

long-stemmed plant. All resulting progeny (F1) were purple-flowered, short-stemmed

plants.

(a) State the dominant and recessive traits, (b) Explain using diagrams, why all the F1

progeny have the same phenotype, (c) Give the expected phenotype ratios if an F1 plant

is selfed.

Linkage

Linked genes as a result do not undergo independent assortment, they tend to be

passed onto the next generation together.

Show the expected genotypes of the progeny for the following cross, AaBb X aabb. (a)

where there is no linkage, (b) where genes are linked A to B and a to b.

The sex chromosomes are the X and Y chromosomes. The X chromosome is much larger

than the Y and thus carries many more genes.

• Sex linked traits are also called X-linked as they are carried on the X chromosome.

There is no corresponding allele on the Y chromosome.

• The recessive phenotype is more common in males, that is they are

more likely to have X-linked traits/diseases.

• Sex linked characteristics include colour-blindness, Haemophilia and

Muscular Dystrophy.

Colour-blindness

Normal individuals can detect three colours of light (red, green and blue).

Linkage means that genes are located on the same chromosomes.

Non-linked genes

Linked genes

Sex Linkage means that a characteristic is controlled by a gene on a sex (or X)

chromosome.

Normal is the dominant trait (N), colour-blindness is the inability to distinguish

between red and green and is the recessive trait (n).

Females can have three genotypes for colour-blindness.

- For a female to be colour blind she must inherit the recessive

allele from both parents, that means the mother must be a

carrier and the father colour-blind

Because the allele for colour vision is carried on the X chromosome males only have one

allele. This means there are only two genotypes possible in males.

- Males only need one recessive allele to be colour-blind so it is

much more likely than in females.

Haemophilia

Haemophilia is a disorder where people cannot properly clot blood and can bruise very

easily and may bleed to death from a very small cut.

As with any other X-linked trait it is very rare in females as they have to inherit two

recessive alleles (carrier mother, haemophiliac father).

Males only have one allele on the X chromosome, thus only need to inherit one recessive

allele (carrier mother).

N is the dominant ‘normal’ trait while n is the recessive ‘disease-carrying’ trait.

Non-nuclear inheritance

Most DNA is contained in the nucleus of the cell, however mitochondria and

chloroplasts (in plants) each contain one single strand of DNA. The DNA in

mitochondria is present in female eggs but is only present in the tail of sperm, hence

mitochondrial DNA is only passed on from the mother (maternal inheritance).

XNXN XNXn XnXn

XNY- XnY-