Extensions to Mendel’s Laws

  • Mendelian traits tend to be the exception

Allele- alternate form of same gene

Loci- location of a particular gene

Chromosome- carries genes

Chapter 5 (pp 95-113)

  1. Mendelian Ratios Wrong?
  2. Mendel’s Ratios
  3. Monohybrid - 3:1
  4. Dihybrid – 9:3:3:1
  5. Lethal allele combinations (in humans many lead to “miscarriages”)
  6. any genotype that causes death
  7. in a population sense it is more specifically any genotype that prevents passage of genes to the next generation

EX. Mexican Hairless Dogs- Hairless is a dominant trait, Homozygous dominant is lethal

  1. Multiple alleles- (several hundred in humans)
  2. Humans have 2 alleles for all autosomal traits however the gene can exist in more than 2 forms
  3. Leads to variation in phenotypes

EX. ABO blood group

  1. Incomplete Dominance-
  2. the heterozygous phenotype is intermediate to the both homozygotes

EX:

Snapdragons-

RR – Red

rr – White

Rr - Pink

Hair Curliness-

CC- Curly

cc – Strait

Cc - Wavy

  1. Codominant-
  2. both alleles are expressed in the heterozygote

EX. Coat Color in Horses

Red and White both homozygous

Roan (both red and white hairs) is heterozygous

  1. ABO blood group
  2. 4 blood types- A, AB, B, O
  3. 3 alleles- IA, IB, i (IO) (I = isoagglutingen = antigen)

Genotype / Antigen Present / Phenotype
IA i / A / A
IA IA / A / A
IA IB / A and B / AB
IB i / B / B
IB IB / B / B
i i / none / O
  1. A and B are dominant to O, but Codominant to each other
  2. Important for compatible blood transfusion

  1. Epistasis-
  2. One gene effects the expression of another gene

EX #1 – Gene for albinism would mask the gene for hair color

Many different types of Epistasis that lead to some variation of the Mendel’s 9:3:3:1 ratio

  1. Duplicate Recessive Epistasis-
  2. must have the presence of 2 genes to express another
  3. EX: must have B and C to express E or e
  4. ratio is 9:7
  5. Dominant Epistasis-
  6. presence of one gene masks the expression of the dominant and recessive forms of another
  7. EX: B masks the expression of E and e (an entire trait)
  8. ratio is 12:3:1
  9. Recessive Epistasis-
  10. presence of one gene masks the expression of only the dominant form of another
  11. EX: B masks the expression of E only (only dominant form of trait)
  12. ratio is 9:4:3
  1. Penetrance and Expressivity – explain the degree of gene expression
  2. completely penetrant - everyone who inherits the combination of alleles has some symptoms
  3. incompletely penetrant - some individuals do not express the phenotype or have no symptoms
  4. Use percentages to describe Penetrance numerically

EX. Polydactyly- (extra fingers or toes)

  1. variably expressive – intensity varies in different people
  2. some people may have an extra digit on every extremity or some may just have a partial digit on one extremity
  1. Pleiotropy - one gene (protein) controls several functions or has more than one effect
  2. Phenocopy – an environmentally caused trait that appears to be inherited

EX. an injury to the pancreas can mimic diabetes

  1. Genetic Heterogeneity – different genes can produce the same phenotype

EX. 132 forms of deafness

  1. Mitochondrial Genes
  2. Mitochondrial genes only come from mother
  3. mothers usually pass all disorders to all offspring
  4. Only DNA fingerprinting that can be done with hard remains (bone / teeth) Trade Center 9/1/01!
  1. Linkage – transmission of genes on the same chromosome
  2. do not follow Mendelian ratios because genes do not independently assort
  3. Recombinant – a series of alleles on a chromosome that differs from the series of either parent
  4. a result of crossing over

Sex Determination and Linkage

(Chp. 6)

  1. Sexual Development
  2. Y chromosome has SRY – sex-determining region of the Y (1990)
  3. absence of SRY leads to female development
  4. Some males that are XX
  5. one X has the SRY
  6. Some females that are XY
  7. Y chromosome lacks the SRY
  8. Sex Chromosomes not always X and Y
  9. Heterogametic sex- two different sex chromosomes (XY)
  10. Homogametic sex – two of the same sex chromosomes (XX)
  11. Birds-
  12. males- ZZ
  13. females - ZW
  14. Y chromosome (Fig. 6.3)
  15. is fairly simple for several reasons
  16. very few genes – 85 genes
  17. No homolog to crossover with
  18. Y has 3 functional groups
  19. pseudoautosomal regions (PAR1 and PAR2)- regions at the tips of the chromosome
  20. may cross over with regions of the X chromosome
  21. protein functions found in both sexes
  22. Bone growth, hormones…
  23. X-Y Homologs- genes found on the Y that are very similar to the X but

not identical as they are in the PAR

  1. Genes that are unique
  2. SRY
  3. Sperm development
  1. X chromosome is much larger than the Y chromosome
  2. X – more than 1,000 genes
  1. Sex Linked Traits
  2. Y-Linked – on the Y chromosome
  3. Rare
  4. Only one clearly defined is infertility (can’t be passed on)
  1. X-Linked – on the X chromosome
  2. in females passed on just like autosomal traits
  3. In males it’s different because only one X exists
  4. Males are Hemizygous- only one set of X-linked genes
  5. X always comes from mom
  1. X-Linked Recessive
  2. always expressed in the male
  3. Expressed in a female homozygote but not the heterozygote
  4. Passed from heterozygote or homozygote mother to affected son
  5. affected female has an affected father and a affected mother or a heterozygote

mother

  1. EX: ichthyosis, color blindness, hemophilia
  1. X-Linked Dominant
  2. expressed in female in one copy
  3. expressed more severely in male
  4. high rate of miscarriage due to early lethality in males
  5. EX: hypertrichosis (extra hair follicles)
  1. X- Inactivation- Fig. 6.13
  2. most of the genes on one X chromosome in each cell are inactivated
  3. which X (from mother or father) is inactivated is random
  4. females express traits from mother in some cells and traits from father in other cells
  5. Barr Body-
  6. the inactivated X chromosome
  7. only present in females
  8. Manifesting Heterozygote-
  9. A carrier of an X-linked trait who expresses the phenotype
  10. due to X-inactivation
  11. Gender Effects on Phenotype
  12. Sex-limited Traits-
  13. a structure or function of the body that is present in only males or only females
  14. ex: horn development, milk yield, beard growth…
  15. genes are transmitted by parents but hormones are not present to express trait
  16. Sex-influenced Traits-
  17. an allele is dominant in one sex and recessive in another
  18. caused by hormonal differences
  19. ex: male pattern baldness (Bb male = bald, bb female = bald)
  20. Genomic Imprinting- (parent of origin) – Fig. 6.15
  21. difference in gene expression of a gene or chromosomal region depending upon whether it is inherited from the father or the mother and depending on male or female
  22. women can have sons and men can have daughters without passing on their sex-specific parental imprints