Chapter 17: Genetic Crosses

GametesAll body cells except reproductive cells are called somatic cells e.g. cheek cells, liver cells etc.

Somatic cells are diploid (2n) and they have a paired set of chromosomes.

To form new organisms diploid cells undergo a type of cell division called meiosis to form gametes.

The gametes in humans are the sperm & egg. These fuse in sexual reproduction to form the fertilised egg.

The zygote normally goes to form a new organism

We have two versions of most genes. These are called alleles. We get one gene (one allele) from each parent. The second gene is a backup in case there is a mistake on the other gene. If both genes have a mistake the person has a genetic disorder.

Genes have a dominant version (given a capital letter e.g. A) and a recessive version (given a small letter e.g. a)

e.g. BB or Bb

e.g. bb

The genotype has 2 letters used to represent the genes e.g. BB, Bb or bb

Dominant is a gene that is stronger than the other gene. It is expressed and stops the other other gene (allele) from working. A capital letter is always used for a dominant gene.

The phenotype is what an organism physically looks like e.g. black or white coat

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

B = Black coat b = White coat

Genotype + environment = phenotype

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

G = Green pod

G = Yellow pod

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

S = Straight Wing

s = Curved wing

Parent Genotypes

Gametes

Progeny genotypes

Progeny phenotypes

Ratio

G = grey body

g = black body

Incomplete Dominance

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

Parent phenotypes

Parent genotypes

Gametes

Offspring Genotypes

Offspring phenotypes

Pedigree StudiesA pedigree is a diagram showing the genetic history of a group of related individuals

Sex DeterminationThe nucleus of each normal body cell (somatic cell) has 46 chromosomes ie. 2n = 46

This consists of 44 non sex chromosomes called autosomes and two sex chromosomes.

The autosomes control features which are independent of whether a person is male or female e.g. eye colour.

The two sex chromosomes are called the X and Y chromosomes. They contain genes that control the gender (sex) of a species.

The X chromosome is longer than the Y chromosome.

Humans

Every individual somatic (non sex) cell nucleus has two sets of chromosomes. If they have XX the person is female and XY they are male.

Note: The male determines the sex of a child

The ratio of males to females should be 1:1

The work of Gregor MendelKnown as as the father of genetics.

Investigated the inheritance of 7 characteristics of peas such as flower colour, stem height etc.

Carried out numerous experiments on pea plants

Removed pollen producing structures from flowers and transferred pollen into flowers by hand.

Covered treated flowers with brown paper bags to prevent any more pollen reaching them.

Collected seeds from plants, grew plants and examined them to see if they had inherited characteristics he was looking for.

Mendel’s First Cross

Mendel’s First Law: Law of Segregation

In this law mendel predicted that a process had occurred which would halve the number of genes (i.e. meiosis).

After fertilisation two gametes fuse to form a single cell called a zygote. Each gamete contains one factor (allele) for each characteristic. So each off spring receives one factor from each parent.

Chromosomal Basis of Mendel’s First Law

Monohybrid & Dihybrid Cross

Monohybrid cross involves the study of a single characteristic e.g. eye colour

Dihybrid cross involves the study of two characteristics at the same time. E.g. plant size (tall or small) and pod colour (green or yellow)

Mendel’s Second Law (Law of Independent Assortment)

The word alleles can be used instead of factors

Chromosomal Basis for Mendel’s Second Law

Parent genotypes

gametes

Progeny genotype

Progeny phenotype

percentage

T = tall plants G = green pods

t = small plants g = yellow pods

Parent Genotypes

Gametes

F1 Phenotype

Percentages

B = Black coat S = Short hair

b = white coat s = Long Hair

Parent Genotypes

Gametes

F1 Progeny Genotypes

F1 Progeny Phenotypes

Percentages

b) P = Purple S = Short

p = Red s = Long hair

a)

Parent Genotypes

Gametes

F2 Progeny

Phenotypes

Percentages/ratio

Answers:

A)

B)

Parent Genotypes

Gametes

F1 Progeny Genotype

F1 Progeny Phenotype

Percentage

b)

The ratio of offspring in linked crosses

Ratio of the genotypes in linked crosses is different from those in non linked crosses.

Sex Linkage

The Sex Chromosomes in Humans are the X and Y Chromosomes.

X Chromosome carries a lot of genes and the Y chromosome is much shorter and only carries a few genes.

Examples of sex – linked Characteristics

1. Colour-blindness

Normal individuals can detect 3 colours of light (red, green & blue). The allele for normal vision is dominant (N). Colour blindness (n) is usually an inability to distinguish red from green.

The gene for colour vision is located on the X Chromosome (X linked)

Females can have 3 distinct genotypes with respect to colour vision

Genotypes are represented as XXNN, XXNn, XXnn

For a female to be colour-blind she must have the genotype XXnn. The incidence of colour-blindness in females is very low at 0.2%

Males have one allele for colour vision and this allele is on the X chromosome. The Y chromosome has no allele for colour vision.

Only two genotypes are possible in males i.e. XNY- or XnY-

Males only need the recessive allele in order to be colour blind.

Males are more likely to be colour blind than females.

Incidence of colour blindness in males is 8%. Most of these males have a degree of colour blindness. Complete colour blindness is very rare in Ireland.

Haemophilia

Bleeding disorder caused by a lack of a particular blood protein.

Symptom is frequent bleeding often into joints.

Without treatment haemophiliacs may bleed to death even after a small cut.

Haemophilia is caused by a gene located on the X chromosome. Male with haemophilia has genotype XYn-

And female with haemophilia is XXnn. Individuals with genotype XXNN or XXNn or XYN- do not have haemophilia

Non – Nuclear Inheritance

Most DNA and genes are located in the nucleus.

Non-nuclear or extra-nuclear genes are present as small circles of DNA in mitochondria or chloroplasts.

Exam Questions