MEIOSIS Meiosis is sex cell division. It consists of: 1. the DNA replicating normally 2. Homologous chromosomes line up independently (and may cross over). 3. A meiotic cell division. 4. A mitotic cell division. This has the effect of halving the chromosome number and forming gametes. Sexual reproduction is important as it greatly increases variation in a species.
CROSSING OVER During meiosis, as the homologous chromosomes line up before the first cell division, part of the neighbouring homologues may swap. The point at which the crossing over occurs is called the chiasma. Instead of two possible gametes, there are four produced. Lab manual pages 105/6, (107 opt)
BASIC GENETIC CROSSES Remember Mendel? And Peas? And Punnet squares? He found that traits were inherited in chunks, called genes. Simple monohybrid (one trait) crosses: A purple pea is crossed with a white flowered plant (P generation). All of the offspring (F 1 generation) are purple. When the resulting plants were crossed he found that there was always a 3:1 ratio in the offspring (F 2 generation). He correctly deduced that: the parents are separately donating their information to the offspring the purple colour is dominant to the white flower (recessive)
The cross of the F1 generation: Gametes Offspring PP is homozygous dominant pp is homozygous recessive Pp is heterozygous Also called “pure breeding” The genotype is a description of the genes contained in the individual The phenotype is a description of its physical appearance (e.g. Purple)
In bears, white ears are recessive to black Momma bear (white) What is the genotype of Momma Bear? Momma bear=__________ Poppa bear (black) What genotypes could Poppa Bear have? Poppa bear=________ or _______ BEARS This is baby bear (white eared). What is his genotype? Baby bear = Can we say something more about Poppa’s genotype?
Incomplete Dominance and Codominance Some alleles (gene forms) are not simply dominant or recessive. In Incomplete Dominance an intermediate phenotype is produced: In Codominance both alleles are expressed at the same time: Lab manual pages 114 and 115
Incomplete Dominance In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes. In crosses involving incomplete dominance, the genotype and phenotype ratios are identical. Examples of incomplete dominance include flower color in snapdragons (right) and sweet peas, where red and white flowered plants cross to produce pink flowered plants. CRCRCRCR CRCwCRCw CwCwCwCw
Flower color in snapdragons exhibits incomplete dominance, with red flowered and white flowered plants crossing to produce offspring with pink flowers. Possible fertilizations Flower Color in Snapdragons Gametes CRCR CRCR CWCW CWCW Pink F 1 offspring CRCWCRCW CRCWCRCW CRCWCRCW CRCWCRCW Red flowerWhite flower Parents CRCRCRCR X CWCWCWCW Pink
CRCwCRCw CRCRCRCR Gametes Possible fertilizations Offspring Snapdragon Backcross Determine the phenotype and genotype ratios of the offspring resulting from a backcross of the F 1 heterozygote to the red parent. 50% of the offspring are red (RR) and 50% are pink (Rr). X Parents Pink flowerRed flower CwCw CRCR CRCR CRCR CRCRCRCR CRCRCRCR CRCwCRCw CRCwCRCw Red Pink
Possible fertilizations Codominance In cases of codominance, both alleles are independently and equally expressed in the heterozygote. Examples include: Roan (stippled red and white) coat color in cattle. A cross between a red bull and a white cow produces all roan offspring. ABO human blood groups. Roa n CRCWCRCW CRCWCRCW CRCWCRCW CRCWCRCW F 1 offspring CRCR CRCR CWCW CWCW Gametes White cowRed bull CRCRCRCR CWCWCWCW Parents X
Possible fertilizations WhiteRoan Red CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW Offspring Codominance in Cattle In a cross between two heterozygous (roan) shorthorn cattle, red, roan, and white offspring are produced in a 1:2:1 ratio. CWCW CRCR CRCR CWCW Gametes Roan cowRoan bull CRCWCRCW CRCWCRCW Parents X
X Roan cow Red bull Parents Possible fertilizations Crosses Involving Codominance In examples of codominance where a true breeding red or white parent is crossed with a roan parent, the offspring will occur in a 1 : 1 ratio of the parental types (i.e. roan and red, or roan and white) Offspring Roa n Red CRCRCRCR CRCWCRCW CRCWCRCW CRCRCRCR Gametes CWCW CRCR CRCR CRCR CRCRCRCR CRCWCRCW
X Roan cowWhite bull Parents Crosses Involving Codominance Possible fertilizations Whi te Roan Offspring CRCWCRCW CWCWCWCW CWCWCWCW CRCWCRCW Gametes CWCW CWCW CRCWCRCW CRCR CWCW CWCWCWCW
Multiple alleles It is possible to have more than 2 alleles for a particular trait. A common example is the ABO blood groups in humans: O is non-functional A forms a protein with A antigen B forms a protein with B antigen A and B are codominant Lab manual pages 116/117 and 120 Lethal genes are ones that cause death in the individual. The lethal gene may be dominant or recessive. In the heterozygous individual there may be some observed difference, e.g. Manx (tailless) cats. Even when dominant the lethal gene may be passed on if it does not have onset until after reproductive age (e.g. Huntington’s). Lethal Genes
Lethal Alleles Lethal alleles are gene mutations that result in a gene product which is not only non-functional, but affects organism survival. Some lethal alleles are fully dominant and are therefore lethal in the heterozygote. Dominant lethal alleles are usually eliminated rapidly, because their expression is fatal. Exceptions occur when the expression of the allele is delayed until after reproductive maturity, as occurs in Huntington disease. In other cases (e.g. Manx cats), the allele mutation results in a viable heterozygote with a recognizable phenotype. Recessive lethal alleles are fatal only in the homozygote since their effect is masked in the heterozygote carrier.
Possible fertilizations Offspring YY Yr yy Yr Not viable Examples of Lethal Alleles When lethal genes prevent full term development of the embryo, offspring are produced in a 2:1 ratio (2 heterozygotes to one normal). In the inheritance of coat color in yellow mice, offspring phenotype ratios depart from the expected Mendelian 3:1 when yellow mice are mated together. About two thirds of the offspring are yellow, and one third are non-yellow (right). A testcross reveals the yellow colored mice to be heterozygotes. Gametes Y yY y X Parents Yy
The average human is heterozygous for 3 to 5 lethal recessive genes. Example: brachydactyly in humans Shortening of the finger bones is caused by a lethal allele; heterozygotes have shorter fingers, but homozygotes for the lethal allele die in infancy from other skeletal defects. Of the offspring of two brachydactylic people, one in four will die in infancy, one half will show brachydactyly, and one in four will be normal (1:2:1 ratio). Incidence of Lethal Alleles X-ray of a normal handBrachydactyly: note the shortened bones
Cats produce a gene controlling spine length and therefore production of a tail. The allele for taillessness (M L ) is incompletely dominant over the allele for a normal tail (M). The Manx allele M L interferes with spinal development and heterozygotes (M L M) have no tail (the Manx phenotype). In M L M L homozygotes, the double dose of the allele produces an abnormal embryo, which does not survive. Possible fertilizations Offspring MMMM L MLMLMLML NormalManx Not viable The Manx Mutation Gametes M MLML M MLML X Parents MM L
Multiple Alleles in Blood The four common blood groups of the human ABO blood group system are determined by three alleles: A, B, and O (also represented in some texts as I A, I B, I O or just i). This is an example of a multiple allele system for a gene. ABO antigens consist of sugars attached to the red blood cell surface. These sugars provide the individual antigenic properties. The alleles code for enzymes that join these sugars together. Allele O produces a non-functional enzyme that is unable to make changes to the basic antigen (sugar) molecule. The other two alleles (A, B) each produce a different enzyme that adds a different, specific sugar to the basic antigen. Any one individual possesses only two alleles and they are expressed equally. RBC
Multiple Alleles in Blood Phenotype (blood group) Genotypes Allele codes for moleculeAntigen Antibodies in serum OOO Precursor Precursor antigen without extra sugar at end None (also called universal donor) AAA, AO acetyl- galactosamine added to precursor BBB, BO galactose added to precursor AB Contains both sugars None (also called universal recipient)
PhenotypeGenotypes Antibodies in serum Results from adding RBCs from groups below to serum from groups at left A AA, AO anti-B BBB, BOanti-A AB none O OO anti-A anti-B Multiple Alleles in Blood Blood donors must be compatible otherwise the red blood cells of the donated blood will clump together (agglutinate) and block capillaries. ABABO
X Blood group: AB Parent genotyp es Blood group: AB Gametes BBAA AB Multiple Alleles in Blood EXAMPLE 1: For both parents with AB blood type, half of the offspring will be the same as the parents (AB), one quarter will be type A and one quarter will be type B. B Blood group s AB A Possible fertilizatio ns Children' s genotype s AB BBAB AA
Blood group: B Blood group: A X Parent genotyp es Multiple Alleles in Blood EXAMPLE 2: Two parents with blood groups A and B respectively, may produce offspring with all four possible blood groups: AB, A, B and O. This may only occur if both parents are carrying the allele for group O. Possible fertilizatio ns Children' s' genotype s AO OOBO AB Gametes OOBA BO AO A O B Blood group s AB
DIHYBRID CROSSES This involves two traits that are not linked (not on the same chromosome). Each of the traits are inherited independently. Lab manual page 113 In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is dominant to Pentagonal. 2 Quarks both EeTt are crossed:
LINKED GENES Linked genes are on the same chromosome. This means that when cell division occurs the 2 genes are very likely to stay together. So where we might expect a offspring phenotype ratio of 1:1:1:1, we actually get something else. Two genes B (Bent) and D (Dark) are linked. For a cross between BbDd and bbdd… Draw the gametes each could form. Draw a punnet square for the cross. Explain these results: Bent Dark: Bent Light: Straight Dark: Straight Light Lab manual page 108 bd BDBbDd BdBbdd bDbbDd bdbbdd B and D (and b and d) are linked. The 1 Bbdd and 3 bbDd individuals are due to crossing over. The different numbers are due to random chance.