Recessive epistasis: recessive allele epistatic to (stands over) other genes when homozygous. One gene masks the phenotypic effects of another.

Transcription

1 Recessive epistasis: recessive allele epistatic to (stands over) other genes when homozygous. One gene masks the phenotypic effects of another. P X F1

2 x F2 AC Ac a c C a c c BBCC BBCc BbCC BbCc cc is epistatic to (masks the effect of) the A gene. Aa c CC a c a c CC a c a c Cc a c a c Cc a c a c cc 9:3:4 = 9:3:3:1 where 3 and 1 have same phenotype.

3 Molecular explanation. Without the first enzyme (C), it doesn t matter what allele of the second enzyme is present. Precursor Cinnamon pigment Agouti pigment Cinnamon pigment forming enzyme Agouti pigment forming enzyme C = Dominant allele makes a normal enzyme c = Recessive allele makes a defective one

4 1/2 A 1/2 AB 3/4 S- 1/4 ss 3/4 S- 1/4 ss =3/8 A =1/8 0 =3/8 AB =1/8 O =1/8 O + 1/8 O = 1/4 O

5 white squash green squash 12:3:1 Anytime dominant B is present = white. Otherwise 3:1

6 Possible molecular explanation. The B allele encodes a killer protein that degrades all pigment. B allele b allele is inert degrades Precursor Yellow pigment Yellow pigment forming enzyme A = Dominant allele makes a normal enzyme a = Recessive allele makes a defective one

7 Both A and B are required to be purple

8 Possible molecular explanations for complementation. A B A B Gene for pigment enzyme

9 The complementation test. If two mutants complement, they are in different genes. If two mutants fail to complement, they are in the same gene.

10 Gene interaction Inheritance pattern A- B- A- bb aa B- aabb ratio Additive Each genotype results in a unique phenotype :3:3:1 Complementary A t l e a s t o n e d o m i n a n t a l l e l e f r o m e a c h o f t w o g e n e s n e e d e d f o r p h e n o t y p e :7 Recessive Epistasis H o m o z y o u s r e c e s s i v e g e n o t y p e a t o n e l o c u s m a s k s e x p r e s s i o n a t s e c o n d l o c u s :3:4 Dominant Epistasis Dominant allele at one locus masks expression at second locus :3:1 Duplicate Genes O n e d o m i n a n t a l l e l e f r o m e i t h e r o f t w o g e n e s n e e d e d f o r p h e n o t y p e :1 Mendel s laws of genotypic segregation have not been broken! Tip: In problems, sum total of all progeny and divide by 16.

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12 Variations of genes with the environment. Penetrance - % of individuals with genotype that exhibit a particular phenotype. Expressivity - degree or intensity with which a phenotype is expressed in an individual. Factors that contribute to penetrance or expressivity

13 all Rr all Rr

14 Rb- Rb- homozygosity causes embryonic lethality, but Rb- Rb+ causes people to be prone to eye cancer. Incomplete penetrance. Only 75% of people with the dominant Rb- allele develop any signs of retinoblastoma. Incomplete expressivity. Some patients develop retinoblastoma in both eyes, some in one.

15 The Rb- mutation causes embryonic lethality when homozygous. If John and Jane both have retinoblastoma, what is the probability that their child will have retinoblastoma? 1/2 Rb+ 1/2 Rb- 1/2 Rb+ Rb+Rb+ Rb+Rb- 2/3 Rb+Rb- 3/4 get 1/4 don t get =1/2 get 1/2 Rb- Rb+Rb- Rb-Rb-

16 Molecular explanation. An allele of the fur color gene that makes an enzyme that is only stable at lower temperatures present at the cats extremities.

17 No clear mendelian phenotypic ratios can be found!

18 These are called continuous traits, or quantitative traits, because they vary over a wide range of values. All of the mechanisms that we discussed contribute to continuous variation. Complex dominance relationships, multiple alleles, and multiple genes interacting in complex ways determine the genetic diversity of life.

19 Chapter 4. Chromosomal theory of inheritance. Mendel: Followed up his work on peas with another plant which had a complex reproductive system Did not observe the nice ratios he had obtained with peas Spent the last ten years of his life fighting taxation on churches Mendel s work was forgotten for 35 years Mendel did not promote his ideas No physical evidence to support his ideas

20 Higher powered microscopes and special stains revealed: Eukaryotic cells contained nuclei, 1880s. Since egg and sperm were believed to contribute equally to offspring (1854), and each contained one nucleus, nuclei probably contained the genetic material. Nuclei contained chromosomes which could be the basis of inheritance. 1902

21 Nuclei were found to contain chromosomes in homologous pairs. Chromosomes are constant in number for every cell in an organism, but vary significantly from one species to another. Karyotype of human; Chromosomes, colored bodies 2n = pairs Each is doubled! 22 autosomes. X and Y sex chromosomes determine sex.

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23 centromere or kinetochore Centromeres can vary in position-close to the middle (metacentric), or toward one end (acrocentric). Chromosomes may also be banded. p (petite) Sister chromatids q Metacentric Acrocentric

24 DNA replication ( S phase) Gap 1 Interphase Sister chromatids are identical to each other

25 Chromosomes are only visible during division (M phase), when they are tightly compacted. Compaction makes it easier to segregate into different cells, without getting tangled.

26 Nuclear division associated with somatic cell division (non-germ cells). Mitotic cell division results in two identical daughter cells. This way all of our cells have the same genetic information. Four major phases of mitosis: Prophase Metaphase Anaphase Telophase Sister chromatid separation followed by replication

27 Prophase Condensation of chromosomes chromosomes can be visualized, sister chromatids are joined at centromeres Nuclear membrane breakdown Metaphase Nuclear splindle forms Chromosomes line up at equatorial plane Homologous chromosomes

28 Anaphase Sister chromatids separate Telophase Nuclear membrane reforms Chromosomes decondense Then cytokinesis generates two cells