Genetic Inheritance Leaving Certificate Biology Higher Level

Genetic Inheritance Humans have 23 pairs of chromosomes –Each pair of chromosomes are what we cell ‘homologous’ – meaning they contain the same genes –Therefore, everyone has two copies of every single human gene – fail-safe mechanism encase one gene in a cell fails there is another to carry on –22 of these pairs are called autosomes –1 pair are called the sex chromosomes and determine the sex of the individual Male: XY Female: XX

Genetic Inheritance Humans have 23 pairs of chromosomes –22 of these pairs are called autosomes –1 pair are called the sex chromosomes and determine the sex of the individual Female: XX Male: XY

Genetic Inheritance Female Karyotype, 46XX Male Karyotype, 46XY

Genetic Inheritance Gamete Formation and Function: –Gamete: a gamete is a haploid sex cell which has to fuse with another sex cell of the opposite sex in order to survive and pass on its genes to form a new individual –Fertilisation: fertilisation is the fusion of two haploid sex cells (gametes) to form a single diploid cell called the zygote

Genetic Inheritance Allele: an allele is a particular form of a gene (can be represented as a letter, e.g. S or s; H or h) Alleles: alleles are different forms of the same gene (e.g. S is the dominant form of the gene whereas s is the recessive form of the gene) Locus: the locus (plural: loci) of a gene is the position is occupies on a chromosome

Genetic Inheritance Genotype: the genotype of an organism refers to its genetic make-up (e.g. Ss) Phenotype: the phenotype of an organism refers to the physical appearance or characteristics of that organism (e.g. Ss can be responsible for a physical appearance or characteristic) –Genotype and environmental conditions together have an effect on the phenotype

Alleles and Loci Before Meiosis MEIOSIS Male heterozygous for brown eyes Female heterozygous for brown eyes MEIOSIS Bb 10 Bb 2n Mendel’s First Law of Segregation

Alleles and Loci After Meiosis Male heterozygous for brown eyes Female heterozygous for brown eyes Bb 10 Bb n n n n SPERM EGGS Half eggs are B and half eggs are b Half sperm are B and half are b

Alleles and Loci After Fertilisation Male heterozygous for brown eyes Female heterozygous for brown eyes b 13 n b bb 2n Phenotype of F1: Blue Eyes FERTILISATION

Genetic Inheritance

Syllabus: –“Study of the inheritance to the first filial generation (F1) of a single unlinked trait in a cross involving: homozygous parents heterozygous parents sex determination”

Homozygous Parents BB Homozygous bb Homozygous x BBbbBBbb BbBbBbBb All offspring have brown eyes { { F1 Progeny Genotype F1 Progeny Phenotype

Heterozygous Parents Bb Heterozygous Bb Heterozygous x BbBbBbBb BBBbBbbb 3:1 – Brown Eyes : Blue Eyes F1 Progeny Genotype { F1 Progeny Phenotype {

Heterozygous and Homozygous Bb Heterozygous bb Homozygous x BbbbBbbb BbBbbbbb 1:1 – Brown Eyes : Blue Eyes { F1 Progeny Genotype F1 Progeny Phenotype {

Genetic Inheritance of eye colour (brown and blue eyes)

Sex Determination Question 8 (page 170): show by diagrams why in humans the father determines the sex of a child. MEIOSIS MaleFemale MEIOSIS X Y X X 2n

Possible male gametes Possible female gametes Sex Determination n YX X X 2n X Y X X FERTILISATION

Incomplete Dominance Incomplete dominance: incomplete dominance is where two homologous alleles are equally expressed and neither allele is dominant over or recessive to the other –The heterozygous genotype produces a phenotype intermediate between those produced by the two homozygous genotypes –An example is flower colour in snapdragons – red flower crossed with white flower produces pink flowered offspring

Incomplete Dominance Parental phenotypes:RED FLOWERWHITE FLOWER Parental genotypes:RRrr Gamete genotypes:RRrr Possible fertilisations: Gamete genotypes:RrRrRrRr Gamete phenotypes:PinkPinkPinkPink

Gregor Mendel Mendel studied the inheritance of seven characteristics of pea plants: 1.Seed shape 2.Seed colour 3.Ripe pod shape 4.Unripe pod colour 5.Flower position 6.Flower colour 7.Height

Gregor Mendel These 7 characteristics were chosen because each has only two clearly contrasting qualities: 1.Seed shape: round/smooth vs wrinkled (RR vs rr) 2.Seed colour: yellow vs green (YY vs yy) 3.Ripe pod shape: inflated vs constricted (II vs ii) 4.Unripe pod colour: green vs yellow (GG vs gg) 5.Flower position: axial vs terminal (AA vs aa) 6.Flower colour: purple vs white (PP vs pp) 7.Height of stem: tall vs dwarf (TT vs tt)

Gregor Mendel Mendel used pea plants because they have several advantages over other plants: –they have a short life cycle –relatively easy to cultivate –could be grown in large numbers –are capable of self-pollination and fertilisation

Gregor Mendel Mendel developed separate populations of pea plants, each a pure breed (homozygous) for a particular quality –e.g. for height, Mendel developed purebred (homozygous) tall pea plants and purebred (homozygous) dwarf pea plants (this took a long time to achieve as Mendel had to check that the purebred tall plants always produced 100% tall offspring and ditto for dwarf pea plants

Gregor Mendel Mendel kept strict records of his results Mendel converted the results of his many crosses into simple ratios that gave him an insight into mechanism of inheritance and led to his two famous laws of genetics

Gregor Mendel’s First Cross Mendel carried out 2 consecutive crosses: TALLxDWARF TTtt Phenotypes: Genotypes: F1 generation: Tt SELF-FERTILISATION x TT ; Tt ; Tt; tt3 tall:1 dwarfF2 generation:

Mendel’s First Law of Genetics: Law of Segregation He repeated this cross for the other six characteristics of pea plants and consistently came up with the same ratio 3:1 Mendel worked backwards and came up with the Law of Segregation

Mendel’s First Law of Genetics: Law of Segregation Law of SegregationLaw of Segregation: each cell contains two factors for each trait, these factors separate during the formation of gametes so that each gamete contains only one factor from each pair of factors. At fertilisation the new organism will have two factors for each trait, one from each parent.

Dihybrid Crosses Having worked out the mechanism governing the inheritance of one characteristic, Mendel then proceeded to study the simultaneous inheritance of two different characteristics, e.g. height and seed shape Again Mendel began his dihybrid cross with purebreds for the characteristics he wanted to study Mendel knew from his monohybrid crosses that tall (T) and round seed (R) are dominant, so dwarf (t) and wrinkled seed (r) are recessive

Dihybrid Crosses Tall, round xDwarf, wrinkled TTRRttrr Phenotypes: Genotypes: F1 generation: TtRr Gametes: TRtr

Dihybrid Crosses Mendel understood the results of the F1 generation from a cross of parents homozygous dominant and homozygous recessive because the only offspring that could be produced from this cross was heterozygous individuals – because each individual could only produce ONE type of gamete due to the fact that they were homozygous for the two traits studied in this cross

Dihybrid Crosses The dihybrid cross between parent plants heterozygous for both traits posed a problem – how are the gametes made from the genotype: TtRr? Mendel’s solution to the problem of gamete formation involving more than one characteristic is Mendel’s Second Law: The Law of Independent Assortment

Mendel’s Second Law of Genetics: Law of Independent Assortment Law of Independent AssortmentLaw of Independent Assortment: members of one pair of factors separate independently of members of another pair of factors at gamete formation

Independent Assortment Gametes TRTrtRtr TRTTRRTTRrTtRRTtRr TrTTRrTTrrTtRrTtrr tRTtRRTtRrttRRttRr trTtRrTtrrttRrttrr Parents:Tall, Round (TtRr) x Tall, Round (TtRr) Gametes:TR; Tr; tR; tr 9/16 Tall, Round 3/16 Tall, Wrinkled 3/16 Dwarf, Round 1/16 Dwarf, Wrinkled recombinants NOTE: The tall, wrinkled (TTrr andTtrr genotypes), dwarf, round (ttRR and ttRr genotypes) and dwarf, wrinkled (ttrr genotypes) progeny are called recombinants because they differ to the parental genotypes and phenotypes 9:3:3:1

Independent Assortment Parents:Tall, Round (TtRr)Dwarf, Wrinkled (ttrr) Gametes:TR; Tr; tR; trtr 1/4 Tall, Round 1/4 Tall, Wrinkled 1/4 Dwarf, Round 1/4 Dwarf, Wrinkled recombinants NOTE: The tall, wrinkled (TTrr andTtrr genotypes) and dwarf, round (ttRR and ttRr genotypes) progeny are called recombinants because they differ from the parental genotypes and phenotypes x x Gametes tr TRTtRr TrTtrr tRttRr trttrr 1:1:1:1

Non-Linked v Linked The genes governing the traits studied by Mendel were found to be ‘non-linked’ meaning that each trait studied was on a separate chromosome to another trait Note: non-linked genes are on different chromosomes and so will undergo independent assortment and therefore are true to Mendel’s Second Law ‘Linked’ alleles (linkage): linked alleles are those genes found on the same chromosome

Linked Genes Linked genes are the genes that are present on the same chromosome Note: genes are said to be tightly linked if they are close together on the same chromosome – tightly linked genes tend not to follow Mendel’s Second Law of Independent Assortment

Linked Genes Non-linked; Genotype: RrTt RRr t T t T r Linked; Genotype: RrTt

Example of Linked Genes In maize: C (coloured seed) is dominant over c (colourless seed) and S (full seed) is dominant over s (shrunken seed) Firstly, a heterozygous coloured, full seed (CsSs) maize plant is crossed with a homozygous recessive colourless, shrunken seed (ccss) maize plant Secondly, two heterozygous coloured, full seed (CcSs) maize plants are crossed Note: the genes for coloured seed and full seed are linked tightly

1.Parent phenotypes:Coloured FullColourless Shrunken 2.Parent genotypes: CcSsccss 3.Meiosis 4.Gamete genotypes: 5.Possible random fertilisations: 6.F1 progenyColouredColourless phenotypes:FullShrunken C S c s c s c s C S c s c s C S c s c s c s x x 1:1

1.Parent phenotypes:Coloured FullColoured Full 2.Parent genotypes: CcSsCcSs 3.Meiosis 4.Gamete genotypes: 5.Possible random fertilisations: CCSSCcSsCcSsccss 6.F1 progenyColouredColouredColouredColourless phenotypes:FullFullFullShrunken C S c s C S c s C S c s C S c s C S c s C S c s c s c s C S C S x x 3:1

Ratio of Offspring Between Non- Linked and Linked Genes Ratio of genotypes of gametes from an non-linked cross are different from those produced by a linked cross Non-Linked Cross Linked Cross RrTtx RrTt PARENTS: GAMETES: RT; Rt; rT; rt RT; rt F1: RRTTRRTtRrTTRrTt RRTtRRttRrTtRrtt RrTTRrTtrrTTrrTt RrTtRrttrrTtrrtt RRTT RrTt rrtt RT; Rt; rT; rtxx RT; rt 3:1 9:3:3:1

Ratio of Offspring Between Non- Linked and Linked Genes Ratio of genotypes of gametes from an non-linked cross are different from those produced by a linked cross Non-Linked Cross Linked Cross RrTtx rrtt PARENTS: GAMETES: RT; Rt; rT; rt RT; rt F1 OFFSPRING: RrTt; Rrtt; rrTt; rrttRrTt and rrtt rtxx 1:1:1:1 1:1

Sex Linkage Sex linkage is where a characteristic is controlled by a gene on an X chromosome Sex-linked genes can also be said to be X-linked The X chromosome carries many more genes (~800 more genes) than the Y chromosome Recessive genotypes for particular traits that are X- linked therefore occur more frequently in males than in females Females have a pair of genes governing each trait – if one gene is faulty, then she has a second one to cover for it However, if a gene is faulty on the X chromosome of a male then he may not have a second one to cover and is more likely to suffer an X-linked genetic defect

Common Sex-Linked Traits Colour vision: gene controlling colour vision has two alleles: N (normal) and n (colour-blind) Blood clotting: gene controlling blood clotting has two alleles: N (normal) and n (haemophiliac) –Haemophilia is the inability to clot blood and a haemophiliac therefore suffers from persistent bleeding if the deficient protein factor needed is not taken

Haemophilia

Haemophilia (cont.) There are three possible female genotypes for a sex-linked trait (e.g. haemophilia) and only two for males: –Females:NN; Nn; nn –Males:N–;n– Heterozygous female for this trait is called a ‘carrier’ – she will pass on this defective allele to 50% of her gametes (egg cells) and thus 50% of her children There are six possible crosses for this trait:

Haemophilia (cont.) FEMALEMALE NNN– NNn– NnN– Nnn– nnN– nnn–

Haemophilia (cont.) Parental phenotypes:Normal (carrier) female Normal male Parental genotypes:X N X n X N Y – Gametes:X N X n X N Y – Random fertilisations: F1 progeny genotypes:X N X N ; X N Y – ; X N X n ; and X n Y – F1 progeny phenotypes:Normal female; Normal male; Normal carrier female; and haemophiliac male XNXN Y–Y– XNXN XNXNXNXN XNY–XNY– XnXn XNXnXNXn XnY–XnY–

Non-Nuclear Inheritance DNA is also found in the mitochondrion – it is also found in the chloroplast The DNA found in these organelles is described as non-nuclear Mitochondria and chloroplasts replicate themselves in a process similar to binary fission NOTE: male gametes only pass on a haploid nucleus at fertilisation whereas the female gamete (egg cell) passes on a haploid nucleus and the cytoplasm – which includes mitochondria and chloroplasts

Non-Nuclear Inheritance (cont.) Therefore, non-nuclear inheritance (i.e. the mitochondrial DNA) is by way of the female gamete ONLY Non-nuclear genes show a non-Mendelian pattern of inheritance