Genetics Honours

Genetics Honours What you will learn in this section The origin of the science of genetics Mendal's experiments Mendal's laws Dihybrid crosses Linkage Sex linkage Non-nuclear inheritance.

Father of Genetics Monk and teacher. Experimented with purebred tall and short peas. Discovered some of the basic laws of heredity. Studied seven purebred traits in peas. Called the stronger hereditary factor dominant. Called the weaker hereditary factor recessive. Presentation to the Science Society in1866 went unnoticed. He died in 1884 with his work still unnoticed. His work rediscovered in 1900. Known as the “Father of Genetics”.

Mendel’s Observations He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds. He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.) He crossed these pure strains to produce hybrids. He crossed thousands of plants and kept careful records for eight years.

Mendel’s Peas In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.) The flower is the reproductive organ and the male and female are both in the same flower. He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the carpel (female sex organ) of another purebred pea plant to form a hybrid or crossbred.

Mendel’s Results Mendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall. (he expected half tall and half short) (discovery of dominance) When these tall offspring were crossed the result was a ratio of 3 tall to 1 short.

Mendel’s Peas

What Did Mendel Find? He discovered different laws and rules that explain factors affecting heredity.

Rule of Unit Factors Each organism has two alleles for each trait Alleles - different forms of the same gene Genes - located on chromosomes, they control how an organism develops

Rule of Dominance The trait that is observed in the offspring is the dominant trait (uppercase) The trait that disappears in the offspring is the recessive trait (lowercase)

Law of Segregation – Mendal's 1st. Law The two alleles for a trait must separate when gametes are formed A parent randomly passes only one allele for each trait to each offspring

Law of Independent Assortment – Mendal's 2nd. Law The genes for different traits are inherited independently of each other.

Questions... How many alleles are there for each trait? What is an allele? How many alleles does a parent pass on to each offspring for each trait

Questions... What do we call the trait that is observed? What case (upper or lower) is it written in? What about the one that disappears? What case is it written in?

Phenotype & Genotype Phenotype - the way an organism looks red hair or brown hair genotype - the gene combination of an organism AA or Aa or aa

Heterozygous & Homozygous Heterozygous - if the two alleles for a trait are different (Aa) Homozygous - if the two alleles for a trait are the same (AA or aa)

Dihybrid vs Monohybrid Dihybrid Cross - crossing parents who differ in two traits (AAEE with aaee) Monohybrid Cross - crossing parents who differ in only one trait (AA with aa)

Questions... What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What is monohybrid crossing?

Example 1 Short hair (L) is dominant to long hair (l) in mice. What is the genotype and phenotype ratio of a heterozygous short-haired mouse crossed with a long-haired mouse?

Example 1: Monohybrid Cross Short hair = dominant = L (LL or Ll) long hair = recssive = l Ll x ll(heterozygote parent = Ll) Punnett Square: Genotype ratio: ½ Ll: ½ ll Phenotype ratio: ½ short hair: ½ long hair L l Ll ll

Monohybrid Crosses Cross that involves one pair of contrasting traits Solve using Punnett Square Try these sample problems: Rr x rr RR x rr Rr x Rr Rr x RR

Dihybrid Crosses Involves two pairs of contrasting traits Pea shape and pea colour Coat length and coat colour in rodents Plant height and flower colour

Example 2 In guinea pigs, the allele for short hair (S) is dominant to long hair (s), and the allele for black hair (B) is dominant over the allele for brown hair (b). What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?

Example 2: Dihybrid Cross Short hair = dominant = SS or Ss Long Hair = recessive = ss Black coat = dominant = BB or Bb Brown coat = recessive = bb SsBb x SsBb SB, Sb, sB, sb and SB, Sb, sB, sb

Example 2: possible genotypes from cross SB Sb sB sb SSBB SSBb SsBB SsBb SSbb Ssbb ssBB ssBb ssbb

Example 2: Phenotypes from the dihybrid cross The possible phenotypes for the parents heterozygotes for both traits are 9/16 Black, short coats 3/16 Black, long coats 3/16 Brown, short coats 1/16 Brown, long coats Ratios are 9:3:3:1

Linkage Mendal's Laws assume the every gene is independent of each other which would meán every gene is on a different chromosome. In reality there are many genes on each chromosome. Genes that are on the same chromosome are siad to be linked. Linkage means that genes are located on the same chromosome. The genes A,B,C,D and E are on the same chromosome, they are said to be linked.

Linkage Under normal conditions genes that are linked remain together during cell division. Where there is linkage Mendal's 2nd. Law does not hold. Linked genes are inherited together and not independently as in Mendal's 2nd law. Linkage means that genes are located on the same chromosome. The genes A,B,C,D and E are on the same chromosome, they are said to be linked.

Example Linkage no Crossing Over The Genes for seed colour (Y) and seed shape (R) in peas are linked. RR and Rr = round seeds rr = winkled seeds. YY and Yy = yellow seeds yy = green seeds. R Y r y Linkage means that genes are located on the same chromosome.

Example Linkage no Crossing Over The Genes for seed colour (Y) and seed shape (R) in peas are linked. Parents heterozygous for both traits. R Y r y R Y r y X Parental Genotypes

Example Linkage no Crossing Over Possible gametes. Note Ry and rY are not possible because of linkage. R Y R Y r y r y X or or

Example Linkage no Crossing Over Possible genotypes RY ry RRYY RrYy rryy Phenotypes 3 : 1 Round/yellow wrinkled/green If there was no linkage there would be 9:3:3:1 ratios as well as Round/green and wrinkled/yellow varieties. MENDEL'S LAW IS NOT OBEYED

Example Linkage and Crossing Over The Genes for seed colour (Y) and seed shape (R) in peas are linked. Parent 1 heterozygous for both traits. Parent 2 homozygous recessive R Y r y r y r y X Crossing over results in some rY and Ry gametes but not in the same numbers as there would be with no linkage at all.

Example Linkage no Crossing Over Possible gametes. Note Ry and rY are now possible because of crossing over. r y r Y R Y R y r y or or or X

Example 2: possible genotypes from cross with crossing over RY Ry rY ry RrYy Rryy rrYy rryy Possible Phenotypes Round/yellow round/green wrinkled/yellow wrinkled/green 48% 1% 1% 48% Not 1:1:1:1 As you would expect if Mendel's 2nd law was obeyed. The new combinations due crossing over are called recombinents

Inheritance Sex Linkage

What is Sex Linkage? In addition to their role in determining sex, the sex chromosomes have genes for many characters. Genes located on a sex chromosome are called sex linked genes. In humans the term usually refers to X-linked characters: genes located only on X chromosomes. Fathers can pass X-linked alleles to their daughters, but not sons. Mothers can pass sex-linked alleles to both sons and daughters.

Recessive alleles If a sex linked trait is due to a recessive allele: A female will express the phenotype only if she is homozygous recessive. If a male receives the recessive allele from his mother he will express the phenotype. Far more males have disorders that are inherited as sex linked recessives than females. Examples: Colour blindness Haemophilia

Red-Green Colourblindness Normal vision Colour blind simulation http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm © 2007 Paul Billiet ODWS

Red-green colour blindness X chromosome has a locus for colour vision with two alleles: XN = Normal colour vision Xn = Red-green colour blindness Y chromosome does not have a colour vision locus. If a male receives the Xn allele he will have impaired colour vision, whereas a female with XNXn will not.

Red-green colour blindness Parental Phenotypes Carrier Female x Normal Male Genotypes XNXn XNY Gametes Offspring 1 Genotypes XN Xn XN Y Female Gametes XN Xn Male Gametes XN XN XN Xn Y XN Y Xn Y Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1 : 1

Haemophilia Haemophilia is sex-linked disease much rarer than red-green colour blindness. Haemophilia is the inability of the blood to clot leading to slow persistent bleeding especially at the joints. About 1 in 7000 males inherit the disease which is potentially lethal. It can be controlled by giving haemophiliacs blood factor 8 transfusions. Haemophiliac females are much rarer. They are unlikely to have children. The onset of menstruation at puberty is often fatal.

Haemophilia Result of a cross betwen a female carrier of haemophilia and a male with haemophilia.

Haemophilia Example of a cross between a carrier female and a normal male

Non-Nuclear Inheritance In eurykaryotic cells mitochondria and chloroplasts are believed to be remnents of prokaryotes that were captured as prey that resisted digestion and a symbiotic relationship. Chloroplasts and mitochrondria can self-replicate and each has a ring of DNA. Their genes code for specialised proteins for respiration or photosynthetic tasks. During sexual reproduction sperm only contribute only a nucleus while the egg supplies a nucleus and cytoplasm. So we only inherit our mitochrondrial DNA from our mothers. Mitrochrondrial DNA is siad to follow a maternal line of inheritance.

Non-Nuclear Inheritance

Mitochondrial DNA comparisons can be used to trace ancestry:

During the Bolshevik revolution, the Tsar’s family was captured and executed.

There are many stories about what happened to their youngest daughter Anastasia Tsar’s Family

Non-Nuclear Inheritance Anna Anderson, claimed she was Anastasia

Non-Nuclear Inheritance Anna Anderson claimed she was Anastasia, but tests of her mtDNA and one of Anastasia’s maternal relatives did not match.

Non-Nuclear Inheritance For more info check out: http://en.wikipedia.org/wiki/Grand_Duchess_Anastasia_Nikolaevna_of_Russia Or the book “Seven Daughters of Eve” by Bryan Sykes