Transmission Genetics

1
Transmission Genetics

• Chapters 14-15 in the textbook
• Focus on Section 14.4-human traits

2
Transmission Genetics-sex linkage

• Genes that show a different pattern of
  inheritance in different sexes
• They are located on the sex chromosomes-not the
  autosomes
• May be dominant, recessive, etc.
• Expression may be affected by autosomal genes

3
Transmission Genetics-X and Y

• X is about 150 Mbp and 1500 genes
• Y is about 57 Mbp and 250 genes (?)
• Women can be homo- or heterozygous for X linked
  genes
• Men are hemizygous
• In women, one of the Xs is not active

4
Transmission Genetics-sex linkage

• Y inherited uniparentally
• X passed down from mom and dad
• X-linked genes easy to discover
• Many disorders

5
Transmission Genetics-sex linkage

• Hemophilia A- Factor VIII blood clotting
  deficiency
• Internal bleeding
• X-linked recessive
• Occurs 1/5000 to 1/10000 male births

6
Transmission Genetics-Fragile X

• Cause of familial mental retardation or fmr
• Cytogenetic Location Xq27.3
• Molecular Location bp 146,801,200 to 146,840,302

FMR1 gene Active in normal brain and other
tissues Inactive in affected individuals 1/2000
male births Symptoms variable-even within
one family
7
Transmission Genetics-X and Y nondisjunction

• X and Y do have some homologous regions
• They have a few common genes
• They can synapse at meiosis
• Failure to separate correctly is called
  nondisjunction
• Nondisjunction of X and Y leads to characteristic
  abnormalities

8
Transmission Genetics-nondisjunction
Gametes at A and B inherit too many or too few
chromosomes. Most nondisjunction gametes are not
viable. But many sex chromosome nondisjunction
gametes do survive.
9
Transmission Genetics-Turners Syndrome

• Nondisjunction sperm leads to 45XO karyotype
• Monosomy
• Phenotype female
• Infertility, short stature, others
• 1/2500
• Note-abnormal X may lead to Turner symptoms in XX
  individuals

10
Transmission Genetics-Trisomy X

• 47XXX karyotype
• 1/1000 births (est.)
• No clinical symptoms
• Normal phenotype, fertile
• Detectable only by karyotype
• We can tolerate variable of X

11
Transmission Genetics-Klinefelters Syndrome

• 47 XXY karyotype
• Male phenotype with some female secondary sex
  characteristics
• Usually sterile
• 1/500 births
• Testosterone therapy

12
Transmission Genetics-Trisomy 21 or Downs
Syndrome

• 47XX21 karyotype
• Mental retardation. Physical abnormalities
• Nondisjunction in mom
• 1/800 births

13
Transmission Genetics-nondisjunction

• Euploid-has normal number and complement of
  chromosomes
• Aneuploid-has abnormal number or complement of
  chromosomes

14
Transmission Genetics-recombination

• Sex-reversed individuals-phenotypic and
  chromosomal sex dont match
• XY females and XX males
• Involve translocation of X and Y material

15
Transmission Genetics-gender determination

• Y carries gene responsible for gender
• SRY gene
• Controls other genes
• Disrupts default (female) pathway

16
Transmission Genetics-gender determination

• SRY not the only gene involved in the pathway
• Many others
• SRY most important

17
Transmission Genetics-pedigree analysis

• Pedigree-formalized picture of family history
• Necessary tool because of ethical limitations
• Developed out of genealogical pedigrees
• Very useful in genetic counseling
• Symbols for people, lines for relationships

18
Transmission Genetics-pedigree analysis

• Sample pedigree

19
Transmission Genetics-pedigree analysis

• Standard symbols
• Used in analysis and counseling

20
Transmission Genetics-pedigree analysis
I II III

• Pedigree-for an autosomal dominant trait

21
Transmission Genetics-pedigree analysis

• Two sample pedigrees for autosomal recessive

22
Transmission Genetics-pedigree analysis

• Sample pedigree-sex linked recessive