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Who’s your mama? Cytoplasmic inheritance • The transmission of cytoplasm differs between sex cells January 18 – Male contribution: Sperm or pollen transfer little or no cytoplasm to the egg – Female contribution: Egg contributes almost all of the cytoplasm to the zygote Organelle chromosomes Organelle chromosomes http://www.erin.utoronto.ca/~w3bio/bio207/index.htm What is in the cytoplasm? • What is in the cytoplasm that could contain DNA? – Mitochondria, chloroplasts • Endosymbiotic hypothesis: – Free living prokaryotes ancestors of chloroplasts and mitochondria invaded plant and animal cells but provide useful function and so a symbiotic relationship developed over time Fungi • The cells of fungi may have many mitochondria (but not chloroplasts) • Neurospora crassa like many fungi is haploid and produces spores • This fungus is a member of the ascomycetes and produces ascospores as the products of meiosis • A zygote inherits its organelles from the cytoplasm of the egg: Maternal inheritance • The pattern of inheritance is not associated with meiosis or mitosis because the organelles are in the cytoplasm not the nucleus • Organelles (Chloroplasts and mitochondria) have circular chromosomes Text p.104 Neurospora crassa Neurospora “sex” • Mating type is a simple form of sex • Mating types are determined by two alleles of a single gene: MAT-A and MAT-a. • Crosses between the two haploid mating types produces diploid meiocytes then ascospores 1 Reciprocal crosses Which parent is ”female”? “Females”: give cytoplasm • Reciprocal crosses of wild type Neurospora with a mutant for growth called poky • The fungi are also carrying either ad+ or ad- (ad is a nuclear gene for ability to grow on adenine deficient medium) “Females”: give cytoplasm • Mitochondria in the first cross are from the cytoplasm of the parent that has a poky phenotype “Females”: give cytoplasm • Mitochondria in the second cross are from the cytoplasm of the female that has a normal phenotype • Mitochondria in the first cross are from the cytoplasm of the parent that has a poky phenotype • Mitochondria in the second cross are from the cytoplasm of the female that has a normal phenotype • Note the nuclear gene ad is a 1:1 ratio ad+ :ad- as expected Nuclear genes Cytoplasmic or Extranuclear • Non-Mendelian inheritance pattern • The results of reciprocal crosses differs depending on one parent in every generation • All progeny both male and female resemble one of the parents (compare this to sex-linkage of nuclear genes) • Extranuclear/cytoplasmic genes cannot be mapped to chromosomes Chloroplasts • Green plants have chloroplasts in their cells • The chromosomes of chloroplasts are circular Text p.55-57 2 Results of crossing flowers White Green Variegated Homoplastic Egg cell ♀ Phenotype of branch bearing eggs (♀) parent Phenotype of branch bearing pollen parent (♂) Phenotype of progeny White White White White Green White White Variegated Green White Green Green Green Green Green Variegated Green Pollen Zygote (2n) cell ♂ White Variegated Green Variegated, green, white, Variegated Green Variegated, green, white, Variegated Variegated Variegated, green, white, Text p.57 Cytoplasmic segregation Cytoplasm of variegated plants Variegated : White Green Variegated • Homoplastic: genetically identical organelles • Heteroplastic: genetically mixed organelles – Also known as cytohets or heteroplasmons Text p.57 Cytoplasmic segregation Human mitochondrial genes • Always inherited maternally • Mutations in mitochondrial genes can give rise to disease if they accumulate, by cytoplasmic segregation or random drift, to such an extent that the affect cell function. Text p.105 3 How is this disorder inherited? A human mitochondrial disease • • • Cytoplasm male sterility • Cytoplasmic male sterility in maize is a mitochondrial trait, sterility results from mitochondrial plasmids (circular DNA in the mitochondria) • The inheritance of mitochondrial alleles is maternal • But the expression depends on the nuclear genotype A child inherits its mitochondria from the cytoplasm of the egg: Maternal inheritance So the condition is always passed on through the mother never the father Occasional unaffected children of an affected mother probably reflect the random assortment segregation in gamete-forming tissue. Cytoplasmic male sterility • Restorer of fertility R is a dominant nuclear gene • R_ male fertility even when cells are homoplastic for mitochondrial male sterility (s) • Male sterility if rr AND homoplastic for s • If rr and normal mitochondria (+) then plant is fertile ♀ RR X rr s + + Male sterile Male fertile s Rr Text ch.3 • Monoecy: male and female flowers are separated e.g. Maize/corn • Tassels have pollen (♂) • Ear shoots have the egg (♀) Cytoplasmic male sterility • Since Restorer of fertility R is a dominant nuclear gene: • ½ of the progeny will be Rr and male fertile even though cells are homoplastic for mitochondrial male sterility (s) • ½ of the progeny will be male sterile since they have rr AND are homoplastic for s ♀ ♂ + s s Plants: monoecious s ♂ s s s rr s Male fertile Male sterile Rr Male fertile Plants: hermaphrodites Plants: dioecious • Many plant species of conifers and angiosperms are hermaphrodite: having both male and female organs • Many plants are dioecious: individual plants are either female or male • Sex determination in the majority dioecious plants is by sex chromosomes • Not all dioecious plants have non-identical chromosomes and those that do have the XY system where the heterogametic sex is male, producing anthers only. s s s s Rr X s s s ½ Male fertile rr s s ½ Male sterile Dioecious plant Text p.49 4 Sex limited traits • Nuclear autosomal genes that are expressed only in one sex – Secondary sex characters – Milk production – Breast development in humans – Male and female plumage in birds Male Female Text ch.3 Course Overview Outline Week 1 2 3 4 5 6 7 8 9 10 11 12 Topic Course objectives and Introduction to genetics Human Pedigrees Patterns of Inheritance: sex-linkage Chromosomal basis of inheritance Changes in chromosome number Gene Mapping Gene to Phenotype Modified Mendelian ratios Model organisms and mutants Genetics of Plant Development (Arabidopsis) Genetics of Animal Development (Drosophila) Behaviour Genetics/Quantitative genetics Chapter Ch. 1 & Ch. 2 Ch. 2 Ch. 2 Ch. 3 Ch. 15 Ch. 4 (Ch. 16) Ch. 6 Ch. 6 Ch. 6 (Ch. 16) Ch. 18 Ch. 18 Ch. 16 + papers Evaluation • Tutorial Quizzes/Assignments worth a total of 15% • 4 Term Tests worth a total of 50%: – January 30 5