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, 46 XX Male Karyotype, 46 XY

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 Male heterozygous for brown eyes 10 2 n B Female heterozygous for brown eyes 10 10 b B MEIOSIS 10 b MEIOSIS Mendel’s First Law of Segregation 2 n

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

Alleles and Loci After Fertilisation Male heterozygous for brown eyes Female heterozygous for brown eyes 13 13 b b n FERTILISATION 13 2 n b 13 b Phenotype of F 1: Blue Eyes

Genetic Inheritance

Genetic Inheritance • Syllabus: – “Study of the inheritance to the first filial generation (F 1) of a single unlinked trait in a cross involving: • homozygous parents • heterozygous parents • sex determination”

Homozygous Parents Homozygous BB Homozygous x bb B F 1 Progeny Genotype F 1 Progeny Phenotype B b b Bb Bb { { All offspring have brown eyes

Heterozygous Parents Heterozygous Bb Heterozygous x Bb B b BB Bb Bb bb { { F 1 Progeny Genotype F 1 Progeny Phenotype 3: 1 – Brown Eyes : Blue Eyes

Heterozygous and Homozygous Heterozygous Bb Homozygous x bb B F 1 Progeny Genotype F 1 Progeny Phenotype b b b Bb Bb bb bb { { 1: 1 – Brown Eyes : Blue Eyes

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. Male X Y X X 2 n Female 2 n MEIOSIS

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

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: Parental genotypes: Gamete genotypes: RED FLOWER WHITE FLOWER RR rr R Possible fertilisations: Gamete genotypes: Rr Gamete phenotypes: Pink R r r Rr Rr Rr Pink

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

Gregor Mendel • These 7 characteristics were chosen because each has only two clearly contrasting qualities: 1. 2. 3. 4. 5. 6. 7. Seed shape: round/smooth vs wrinkled (RR vs rr) Seed colour: yellow vs green (YY vs yy) Ripe pod shape: inflated vs constricted (II vs ii) Unripe pod colour: green vs yellow (GG vs gg) Flower position: axial vs terminal (AA vs aa) Flower colour: purple vs white (PP vs pp) 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: Phenotypes: TALL Genotypes: TT F 1 generation: Tt x DWARF tt Tt x Tt Tt SELF-FERTILISATION F 2 generation: TT ; Tt; tt 3 tall: 1 dwarf

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 Segregation: each cell contains Segregation 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 Phenotypes: Tall, round Genotypes: Gametes: F 1 generation: x Dwarf, wrinkled TTRR ttrr TR tr Tt. Rr

Dihybrid Crosses • Mendel understood the results of the F 1 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: Tt. Rr? • 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 Assortment: Assortment members of one pair of factors separate independently of members of another pair of factors at gamete formation

Independent Assortment Parents: Gametes Tall, Round (Tt. Rr) x Tall, Round (Tt. Rr) TR; Tr; t. R; tr TR Tr t. R tr 9: 3: 3: 1 TR TTRr Tt. RR Tt. Rr 9/16 Tall, Round Tr TTRr TTrr Tt. Rr Ttrr 3/16 Tall, Wrinkled t. R Tt. Rr tt. RR tt. Rr 3/16 Dwarf, Round tr Tt. Rr Ttrr tt. Rr ttrr 1/16 Dwarf, Wrinkled NOTE: The tall, wrinkled (TTrr and. Ttrr genotypes), dwarf, round (tt. RR and tt. Rr genotypes) and dwarf, wrinkled (ttrr genotypes) progeny are called recombinants because they differ to the parental genotypes and phenotypes

Independent Assortment Parents: Gametes: Tall, Round (Tt. Rr) x x TR; Tr; t. R; tr Dwarf, Wrinkled (ttrr) tr Gametes tr TR Tt. Rr 1/4 Tall, Round Tr Ttrr 1/4 Tall, Wrinkled t. R tt. Rr 1/4 Dwarf, Round tr ttrr 1/4 Dwarf, Wrinkled 1: 1: 1: 1 NOTE: The tall, wrinkled (TTrr and. Ttrr genotypes) and dwarf, round (tt. RR and tt. Rr genotypes) progeny are called recombinants because they differ from the parental genotypes and phenotypes

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: Rr. Tt R Linked; Genotype: Rr. Tt r R T t r T t

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 (Cs. Ss) maize plant is crossed with a homozygous recessive colourless, shrunken seed (ccss) maize plant • Secondly, two heterozygous coloured, full seed (Cc. Ss) maize plants are crossed • Note: the genes for coloured seed and full seed are linked tightly

1. Parent phenotypes: 2. Parent genotypes: x x Coloured Full Cc. Ss C c S Colourless Shrunken ccss c c s s s 3. Meiosis C c c 4. Gamete genotypes: S s s 5. Possible random fertilisations: 6. F 1 progeny phenotypes: C c c c S s s s Coloured Full Colourless Shrunken 1: 1

1. Parent phenotypes: 2. Parent genotypes: x x Coloured Full Cc. Ss C c S Coloured Full Cc. Ss C c s S s 3. Meiosis C c 4. Gamete genotypes: S s 5. Possible random fertilisations: 6. F 1 progeny phenotypes: C C C c c c S S S s s s CCSS Coloured Full Cc. Ss Coloured Full ccss Colourless Shrunken 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 PARENTS: Rr. Tt x Linked Cross Rr. Tt GAMETES: RT; Rt; r. T; rt x RT; Rt; r. T; rt F 1: RRTT RRTt Rr. TT Rr. Tt RRTt Rr. TT RRtt Rr. Tt rr. TT Rrtt rr. Tt 9: 3: 3: 1 Rr. Tt x Rr. Tt Rrtt rr. Tt rrtt RT; rt x RT; rt RRTT Rr. Tt 3: 1 Rr. Tt rrtt

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 rrtt PARENTS: Rr. Tt x GAMETES: RT; Rt; r. T; rt x rt F 1 OFFSPRING: Rr. Tt; Rrtt; rr. Tt; rrtt 1: 1: 1: 1 Linked Cross rrtt Rr. Tt x RT; rt x rt Rr. Tt and rrtt 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: – Males: NN; Nn; nn 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. ) FEMALE 1. NN N– 2. NN n– 3. Nn N– 4. Nn n– 5. nn N– 6. nn n–

Haemophilia (cont. ) Parental phenotypes: Normal male Parental genotypes: Normal (carrier) female XN X n Gametes: XN XN Random fertilisations: Xn XN Y – Y– XN X NX N X NY – Xn F 1 progeny genotypes: XN X NX n X n. Y – XNXN; XNY–; XNXn; and Xn. Y– F 1 progeny phenotypes: Normal female; Normal carrier female; and haemophiliac male Y–

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