Inheritance Lesson 3

Lesson 3-Learning Intention True breeding F1 offspring F2 offspring

What we know so far.. An organism’s PHENOTYPE is the characteristic which is in the PHYSICAL APPEARANCE Each characteristic is controlled by TWO GENES. Each copy of the gene is called an ALLELE. The two genes which control the characteristic is called the GENOTYPE

What could the genotype of a true breeding organism be? An organism is said to be TRUE BREEDING if, when crossed with another organism of the same strain, it always produces offspring of exactly the same kind What could the genotype of a true breeding organism be?

True Breeding Organisms Contain 2 genes which are the same for that characteristic- HOMOZYGOUS. Homozygous - 2 identical alleles for any particular characteristic (2 identical forms of the same gene).

Example The invisible woman has super strength- her genotype is HOMOZYGOUS DOMINANT (SS) Superman is also HOMOZYGOUS DOMINANT for super strength (SS) Both parents can only pass on the gene for super strength to their offspring (as that is all they have) All offspring will possess the same genotype as their parents for super strength (SS)

Heterozygous organisms Organisms that have two DIFFERENT alleles are heterozygous (one dominant and one recessive) The dominant gene HIDES the recessive gene and so the dominant gene will appear in the PHENOTYPE (appearance) The organism can still pass on the recessive allele to offspring

Homozygous and heterozygous: copy If an organism possesses 2 identical forms of the same gene, the organism is said to be HOMOZYGOUS and is true breeding. If an organism possesses 2 different forms of a gene, it is said to be HETEROZYGOUS.

Monohybrid crosses Only one characteristic (MONO) is considered Alleles represented by letters- capital for the dominant gene and small letter for recessive

what is the parent’s GENOTYPE? Example For example: - in peas, round seed is dominant to wrinkled seed. This can be represented in symbols: Round = dominant allele = R Wrinkled = recessive allele = r 2 true breeding parents- one has wrinkled seeds and one has dominant seeds what is the parent’s GENOTYPE?

First generation offspring Parents round seeds x wrinkled seeds (true breeding) Genotype   Alleles What would be the possible genotype of any offspring?

First generation offspring Parents round seeds x wrinkled seeds (true breeding) Genotype RR rr   Alleles What would be the possible genotype of any offspring? All F1 offspring would be round seeds Rr (heterozygous) R r R r

Second generation offspring The seeds from the first generation offspring were self crossed F1 self-cross round seeds x round seeds Genotype Gametes

Second generation offspring The seeds from the first generation offspring were self crossed F1 self-cross round seeds x round seeds Genotype Rr Rr Gametes What would the possible genotypes be for these offspring? R r R r

Second generation offspring The seeds from the first generation offspring were self crossed F1 self-cross round seeds x round seeds Genotype Rr Rr Gametes What would the possible genotypes be for these offspring? RR : Rr : Rr : rr Phenotype ratio : 3 round seeds : 1 wrinkled seed R r R r

Sex determination The sex chromosomes determine the gender of an individual Two types of sex chromosome- X and Y Males have an X and a Y (XY) Females have two Xs (XX)

Sex Cells Female sex cells (eggs) can only pass on an X chromosome Male sex cells (sperm) can contain either an X or a Y Fertilisation is a RANDOM process The egg may be fertilised by an X or a Y sperm

Sex determination If egg is fertilised by an X sperm- genotype is XX and zygote develops into a girl If egg fertilised by a Y sperm- genotype XY and zygote develops into a boy

Sex Determination: copy The sex chromosomes (which are inherited from the parents) determine the gender of an individual. There are 2 types of sex chromosome; X and Y. Males have an X and a Y chromosome (XY genotype) Females have two X chromosomes (XX genotype) Females can only pass on an X chromosome in their eggs. There is a 50/50 chance of the egg being fertilised by an X or Y sperm.

Past paper questions

Past paper questions

Past paper questions

Past paper questions

Past paper questions