Lake Michigan College Genetics 212 Mendel Page

Lake Michigan College GENETICS (212)

[Dihybrid] [Codominance] [Sex linkage] [Pedigrees]

MENDELIAN GENETICS

Learn about Gregor Mendel at the Mendel Web
Mendel's Law of Segregation [ alleles separate in gamete formation (meiosis)]
Monohybrid crosses. See the the interactive Pea Experiment
Necessary definitions [see terms]
- phenotype and genotype
- genes and alleles
- heterozygous and homozygous
Mendel's Principles of Dominance and Recessiveness
Mendel's Test Cross [heterozygous to homozygous recessive]
- Monohybrid (Aa X aa) results in 1:1 phenotype ratio
- Dihybrid (AaBb X aabb) results in 1:1:1:1 phenotype ratio
Rules of probability
- Probability is chance that an event will occur
- Probability of one gene segregating from a pair into gamete is 1/2.
- See actual outcomes in coin flips
- For Probability Product Rule try probability calculator
Solving with Punnett Squares [for monohybrids]
Dihybrid crosses
- Mendel's Law of Independent Assortment [alleles for different traits are distributed to gametes independent of one another]
- AaBb X AaBb = 9A_B_ : 3A_bb : 3aaB_ : 1aabb
- See for 9:3:3:1 phenotype ratio Probability Product Rule
- Mendelian genetics tutorials
Classical genetics expanding from Mendel
- Incomplete dominance & Codominance
  - Palomino horse
  - Codominance [sickle cell anemia]
- Multiple alleles [ABO blood types]
- Monohbrid cross problem sets at the University of Arizona
- Law of Independent Assortment and Linked Genes
  - Dihybrid crosses - Dihybrid Cross problem sets
  - Gene Interactions [chicken comb types]
- Mechanism of epistasis
- Epistasis example in coat color or horses
- Multiple alleles
  - ABO blood types
SEX CHROMOSOMES & SEX DETERMINATION
- Chromosomal Theory developed by Thomas Hunt Morgan [Nobel Prize 1933]
  - Genes located on chromosome
  - Genes behave as chromosomes during meiosis
- Euchromatin [DNA is active] and heterochromatin [transcriptionally inactive]
- Chromosomal basis of Sex
  - XX female, XY male [most mammals, humans and fruit flies]
  - ZZ male, ZW female [most birds, some fish and reptiles]
  - XX female, XO male [insects]
- Dosage compensation [XX females, have only one active X]
  - Barr body only in the nucleus of female's cells [inactivated X chromosome]
  - Lyon hypothesis [Mary F. Lyon]
  - Heterozygous females show mosaicism
    - Random inactivation of either chromosome (X^AX^a)
    - Calico cat
    - Demonstration of X inactivation mosaicism
- Sex linked Inheritance
  - Genes found on the X chromosome [sex-linked]
  - Thomas H. Morgan discovers sex-linked genes in fruit flies.
  - XY males are hemizygous [one allele on X and non on Y chromosome]
  - X linked inherited human conditions are rare. Recessive alleles are responsible for the following.
    - hemophilia
    - red-green colorblindness
    - EDA Anhidrotic ectodermal dysplasia
    - Testicular feminization syndrome XY females
- Sex linked inheritance problems
- Autosomal trait due to genes on non-sex chromosomes
  - Two alleles are carried in both males and females
- Sex-limited & Sex-influenced inheritance may be due to autosomal or x-linked genes.
  - Sex influenced trait is conditioned by gender of the indivual
    - Male pattern baldness The Adams family
    - Allele is dominant in men but recessive in woman
  - Sex-limited trait is expressed in only one sex.
    - Breasts in women and facial hair in men
    - Feather pattern and color in birds
- The Y chromosome contains the SRY gene which is the testis determining factor.
- Pedigree Analysis [Deduction of inheritance from family tree]
  - Pedigree of an X-linked recessive trait: Colorblindness
  - Hemophilia the X-linked recessive "royal disease".
  - Pedigree of sex linked trait in royal family
  - Try the Pedigree Quizzes (javascript): [ Pedigree 1 quiz] [ Pedigree 2 quiz] [ Pedigree 3 quiz]
Two terms that describe variation in the expression of genes
- Penetrance and Expressivity
- Penetrance - the frequency of the expression of an allele
  - Most genotypes complete penetrance [100% penetrant] all with genotype show phenotype
  - Incomplete penetrance = If genotype 90% penetrant - only 90% with genotype show phenotype
- Expressivity - genotype penetrant but shows variable degrees of expression
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Revised October 10, 2002
Copyright ©: 1999-2002 Lake Michigan College
Comments To: camelet@lakemichigancollege.edu

Lake Michigan College

GENETICS (212)

MENDELIAN GENETICS

SEX CHROMOSOMES & SEX DETERMINATION