2What 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.
3e.g. Pea plants can either be tall or dwarf. xDwarfxTallTallTallDwarfTallDwarfTallDwarf
4PhenotypesFor 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..
5Phenotypes= male= female= ability to roll tongue= inability to roll tongueGrannyGrandadDadAuntMumUncleBrotherSisterSince each organism receives 2 pieces of information for this characteristic, we now need to work out the GENOTYPE.
6Genotypes The GENOTYPE is the set of genes the organism possesses. RR
7GenotypesThe 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 ttIf they are different, we say they are HETEROZYGOUS e.g. Tt
8Alleles Different forms of the same gene are called ALLELES. E.g. tall and dwarf are different alleles of the same gene.
9ChromosomesChromosomes 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.
10Gamete ProductionChromosomes 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.
11Genetic CrossWhen 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.
12Genetic CrossThis means a plant with the genotype TT will have the phenotype TALLThe plant with the genotype Tt will have the phenotype TALLThe plant with the genotype tt will have the phenotype dwarf.
13Ratios of PhenotypesThe 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.
14Punnett SquareT = tall and t = dwarfIf we cross a homozygous tall with a dwarf plant the following punnett square would be produced.Gametes for tall parentGametesTtTtGametes for dwarf parentIn this cross ALL the offspring are Tt and are therefore, tall.
15If both plants are heterozygous, what would the square look like? Punnett SquareIf both plants are heterozygous, what would the square look like?GametesTtTtTTTtTttt
16And so, half of the offspring will be female and the other half male. Sex ChromosomesSex in humans is determined by the sex chromosomes. Females have 2 X chromosomes (XX) and males have an X and a Y chromosomes (XY).GametesXXYXXXXXYXYAnd so, half of the offspring will be female and the other half male.
17Monohybrid Crosses We always set out a cross the same way: Parents (P) Black x AlbinoBBbbGametesB or Bb or bThe gametes for one parent go along the top.GametesBbBbBbThe gametes for the other parent go down the side.BbBbWe can then complete the square.The results of this cross are that all the F1 are black.
18Monohybrid Crosses If we now cross 2 of the F1 generation: Parents (P) Black x BlackBbBbGametesB or bB or bThe gametes for one parent go along the top.GametesBbBbBBBbThe gametes for the other parent go down the side.BbbbWe can then complete the square.The results of this cross are that 3 of the F1 are black and 1 is albino.
19Monohybrid Crosses Now try one of your own: Parents (P) Black x Albino BbbbGametesB or bb or bThe gametes for one parent go along the top.GametesBbbBbbbThe gametes for the other parent go down the side.BbbbWe can then complete the square.The results of this cross are that half of the F1 are black and half are albino.
20Monohybrid CrossesYour 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.