Inheritance

CELL DIVISION & INHERITANCE

Cell division

For an organism to grow, cells must multiply.

To obtain new cells with the same characteristics all the information in the old cell must pass to the new cells.

Information is stored in the nucleus in a long molecule called the DNA.

Before cell division a copy of the DNA is made; this is called REPLICATION.

Chromosome: part of the DNA, containing genes, seen during cell division.

Types of cell division

Takes place in all body cells except ovaries & testis

Produce 2 daughter cells Daughter cells have full

set of chromosomes (diploid)

One division only No crossing over Parent and daughter

cells are identical

Takes place only in ovaries & testis

Produce 4 daughter cells Daughter cells have half

set of chromosomes (haploid)

2 consecutive divisions Crossing over takes

place Parent and daughter

cells are non-identical.

mitosis meiosis

Gene: a part of a chromosome that codes for a certain character.

A gene is a segment of DNA carrying a recipe for making a certain molecule.

An allele: is a one of different forms of a gene.

Example there are 2 different genes for petal colour in a certain flower, one for white one for red.

Each chromosome holds one allele. If both alleles are similar = homozygous If alleles are different = heterozygous

Some alleles are stronger than others, they are dominant, we give them a capital letter e.g. R for red petals or T for tall plants.

Weaker alleles are given a small letter r for white petals or t for short plants.

Dominant allele: an allele that shows its character in a homozygous or heterozygous pairs.

Recessive allele: an allele that shows its character only in homozygous pairs.

If a plant has one of these pairs of alleles RR, Rr, rr, what petal colours will it have?

RR: Rr: rr:

Predicting genotypes

Mendel when studying his pea plants observed that 75% of the plants were tall while 25% were short.

If a plant was short which pair of alleles could it have?

If a plant was tall which pair of alleles could it have?

Human earlobe

Humans have either free or fixed earlobes. Count number of free and fixed earlobes in class,

which character is dominant?

Choose a letter to represent the dominant allele= Choose a letter to represent the recessive allele= Which pair of alleles do you think you have? Genotype: the type of alleles that an organism has. Phenotype: the observed character that is

influenced by the genes.

Free earlobes

Fixed earlobes

Genetic diagrams

Parent Phenotype

Parent Genotype

Gametes

Offspring genotype Offspring phenotype Ratio

Punnet square

r

r

R R

Rr Rr

Rr Rr

Parent genotype

Parent genotype

RR

rr

Inheritance of gender

Male

Female

What is the chance a pregnant lady gives birth to a male child? Which parent is responsible for the child being male or female?

X Y

X XX XY

X XX XY

Your homework!

Try to find out your genotype for earlobe character. You will need to observe your parents’ earlobes.

Draw a Punnet square to show how you inherited your free or fixed earlobe.

How many of your brothers or sisters have free or fixed earlobe? Does it match the ratio you got using the Punnet square?

Codominance

For some characteristics alleles are neither dominant or recessive; they are codominant; which means both alleles show their effect on the organism.

This results in an organism having a characteristic in between. For example if alleles for red petal colour and white petal colour were codominant, a heterozygous plant will have …………… petals.