INHERITANCE What is Inheritance? G R Davidson

What is Inheritance? Inherited characteristics are determined by genetic information received from parents. This information is passed on in the genes on the chromosomes. Each organism receives 2 pieces of genetic information about each characteristic, one from each parent. These may be different or the same.

e.g. Pea plants can either be tall or dwarf. x Dwarf x Tall Tall Tall Dwarf Tall Dwarf Tall Dwarf

Phenotypes For each inherited characteristic an organism possesses there may be a number of different forms. These forms are called PHENOTYPES. The phenotype describes the appearance of the inherited characteristic, e.g. flower colour may be red, white, etc..

Phenotypes = male = female = ability to roll tongue = inability to roll tongue Granny Grandad Dad Aunt Mum Uncle Brother Sister Since each organism receives 2 pieces of information for this characteristic, we now need to work out the GENOTYPE.

Genotypes The GENOTYPE is the set of genes the organism possesses. RR

Genotypes The way in which inherited characteristics are passed on from parents to offspring, follows a pattern. Genetics is the study of these patterns. If the two genes in a pair are identical, we say they are TRUE BREEDING or HOMOZYGOUS e.g. TT or tt If they are different, we say they are HETEROZYGOUS e.g. Tt

Alleles Different forms of the same gene are called ALLELES. E.g. tall and dwarf are different alleles of the same gene.

Chromosomes Chromosomes are thread-like structures found in the nucleus of a cell, which carry inherited information. Each normal body cell has 2 matching sets of these chromosomes. In humans there are 23 pairs of chromosomes.

Gamete Production Chromosomes are passed on to the next generation in the sex cells (gametes). Each sex cell will contain only 1 of each pair of chromosomes – therefore will only have 23 chromosomes. This allows it to join up with (fertilise) another gamete and make the numbers back up to 23 pairs.

Genetic Cross When we study a characteristic, the first thing we do is assign it a letter e.g. black would be B or tall T. Each characteristic has 2 forms,e.g. tall and dwarf. One of these forms will be DOMINANT over the other, which we say is RECESSIVE. We assign the dominant trait the capital letter and the recessive trait the small letter. E.g. tall is dominant to dwarf, therefore tall will be T and dwarf will be t.

Genetic Cross This means a plant with the genotype TT will have the phenotype TALL The plant with the genotype Tt will have the phenotype TALL The plant with the genotype tt will have the phenotype dwarf.

Ratios of Phenotypes The simplest genetic crosses only involve one characteristic and these are called MONOHYBRID CROSSES. The patterns of inheritance can be followed in a Punnett Square. E.g.

Punnett Square T = tall and t = dwarf If we cross a homozygous tall with a dwarf plant the following punnett square would be produced. Gametes for tall parent Gametes T t Tt Gametes for dwarf parent In this cross ALL the offspring are Tt and are therefore, tall.

If both plants are heterozygous, what would the square look like? Punnett Square If both plants are heterozygous, what would the square look like? Gametes T t T t TT Tt Tt tt

And so, half of the offspring will be female and the other half male. Sex Chromosomes Sex in humans is determined by the sex chromosomes. Females have 2 X chromosomes (XX) and males have an X and a Y chromosomes (XY). Gametes X X Y XX XX XY XY And so, half of the offspring will be female and the other half male.

Monohybrid Crosses We always set out a cross the same way: Parents (P) Black x Albino BB bb Gametes B or B b or b The gametes for one parent go along the top. Gametes B b Bb Bb The gametes for the other parent go down the side. Bb Bb We can then complete the square. The results of this cross are that all the F1 are black.

Monohybrid Crosses If we now cross 2 of the F1 generation: Parents (P) Black x Black Bb Bb Gametes B or b B or b The gametes for one parent go along the top. Gametes B b B b BB Bb The gametes for the other parent go down the side. Bb bb We can then complete the square. The results of this cross are that 3 of the F1 are black and 1 is albino.

Monohybrid Crosses Now try one of your own: Parents (P) Black x Albino Bb bb Gametes B or b b or b The gametes for one parent go along the top. Gametes B b b Bb bb The gametes for the other parent go down the side. Bb bb We can then complete the square. The results of this cross are that half of the F1 are black and half are albino.

Monohybrid Crosses Your answer should always have the parental genotypes. Your answer should always have the parental gametes. Your answer should always have a completed punnett square. Your answer should always have the F1 phenotypes and the ratio they occur in.