Transcript Barron`s Ch 7 ppt Heredity
Chapter 7: Heredity
• • • • • • • Overview of this chapter: Mendelian laws Manipulation of Mendelian laws o Backcross/Testcross o Monohybrid/Dihybrid Cross Beyond Mendelian Inheritance o o Incomplete Dominance/Codominance Multiple Alleles Gene Interactions o Pleiotropy/Epistasis Linked Genes, Cross-over, Linkage Mapping Barr Body Mutations o Gene/Chromosomal Mutations, Nondisjunction
Gregor Mendel
The father of modern genetics is Gregor Mendel - Austrian monk, in 1850s - Bred garden peas in order to study patterns of inheritance - Theory of genetics is one of particulate inheritance • • • - Mendelian Laws: Law of Dominance Law of Segregation Law of Independent Assortment
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Law of Dominance
When two organisms, each
- Mendel’s first law: Law of dominance
homozygous (pure)
- The trait that
for two opposing remains hidden is traits are crossed, known as the the offspring will recessive trait (g) be hybird (carry two different alleles; Gg) but will exhibit only the dominant trait (G)
Law of Segregation
- During formation of gametes, the two traits carried by each parent separate
- Best exemplified by monohybrid cross - Trait that’s not evident in either parent appears in F1 generation Thank you Jackie for the amazing paint skills.
Backcross or Testcross
- Way to determine genotype of an individual plant or animal showing only dominant trait - Ex.) individual (B/_) is crossed with homozygous recessive (b/b) - If individual is in fact homozygous dominant, all offspring will be B/b and show dominant trait. - No offspring showing recessive trait.
- If individual is hybrid (B/b) - one half of offspring show recessive trait - Therefore if any offspring show recessive trait, parent must be hybrid.
Law of Independent Assortment
- Applies when cross is carried out between two
individuals hybrid for two or more traits are not on
the same chromosome. - called dihybrid cross.
- Law states that during gamete formation, alleles for one trait (e.g. height), segregate independently from the alleles of a gene for another trait (e.g. seed colour) - Only factor that determines how these alleles segregate (assort) is how the homologous pairs line up in metaphase of meiosis I, which is random. - However, if the genes are linked then they will not assort independently.
The Dihybrid Cross
- Cross between two F1 plants is called a dihybrid cross - Cross between individuals that are hybrid for two different individuals that are hybrid for two different traits (e.g. height and seed colour) This cross can produce four different types of gametes: TY, Ty, tY, ty (dihybrid cross) - Many different genotypes possible in the resulting F2 generation.
- Phenotype ratio of the dihybrid cross, 9:3:3:1, 9 tall, yellow; 3 tall, green; 3 short, yellow; and 1 short, green. F2: 9/16 tall, yellow; 3/16 tall, green; 3/16 short, yellow; 1/16 short, green.
Incomplete Dominance Codominance both traits show
-Incomplete dominance is characterized by
blending.
- Neither trait is dominant, thus the convention for writing the genes uses a different convention: all capital letters.
Multiple Alleles
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When there are more than two allelic forms of a gene
Gene Interactions
- Pleiotropy: Ability of single gene to affect organism in several ways. - e.g. autosomal recessive disease; cystic fibrosis. - Epistasis - two separate genes control one trait -
but one masks expression of other gene
- Gene that masks the expression of the other gene is epistatic to the gene that's masked
Polygenic Inheritance
Polygenic: genus that vary along a continuum, e.g. skin colour, height, hair colour.
Genes and the Environment
- Environment can alter expression of genes - Development of intelligence is result of interaction of genetic predisposition and environment - Genus from previous generations “learned” of a way to “better” do something
Linked Genes
- genes on same chromosomes - Tend to be inherited together and do not assort independently (unless separated by cross-over)
Sex-Linkage
-Of 46 human chromosomes - 44 are autosome and 2 are sex chromosomes -traits carried on X chromosome are sex-linked -females (XX) inherit two copies of sex-linked genes -males (XY) inherit only one copy of sex-linked gene -mutated X-linked gene is: X -can lead to sex-lniked traits such as colour blindness, hemophilia and Duchenne muscular
dystrophy
Important facts about sex-linked traits
- All daughters of affected fathers are carriers!
-Remember: Sex-linked traits are located on X chromosomes.
- Son has 50% percent chance of inheriting sex-linked trait from carrier mother - No “carrier" state for X-linked traits in males.
- If male has gene, he will express it!
- Uncommon for female to have recessive sex-linked condition (for it to happen, she must inherit mutant gene from both parents)
Cross-over and linkage mapping
- The farther two genes on chromosomes - more likely to be separated by cross-over during meiosis.
- When cross-over/recombination occur, one can see chiasmata (physical bridge built around point of exchange)
- Result of cross-over = recombination - Major source of variation!
- Map unit: distance within which recombination occurs 1 percent of the time.
- Tells of order of linked gene on chromosome (pretty useful for mapping genome) Ex) 1) Genes A, B, D are linked 2) Cross-over or recombination frequencies for B and D is 5%, B and A is 30%, D and A is 25% 3) Draw linkage map from this (name BDA or ADB)
Important Tools to Know Recombination Frequency:
Number of recombinants ------------------------------------- x 100 Total number of offspring
Pedigree:
X-Inactivation - Barr body
- In development of embryo in female mammal, one of X
chromosome is inactivated in every somatic cell
- Inactivation occurs randomly - Results in genetic mosaic - Some cells have one X inactivated, some cells have other X inactivated, therefore all cells of female mammals not identical.
- Inactivated chromosome condenses into dark spot of chromatin - Barr body
Remember: All female body cells have one Barr Body. Normal male cells have none.
Mutations
Mutations: any changes in the genome - Two types: gene mutations/chromosomal mutations - Chromosomal mutations can be seen with karyotype -Chromosomal aberrations: Deletion, Inversion,
Translocation, Polyploidy, (Duplication)
Some examples....
Genetic Disorder
Cystic fibrosis Huntington's disease Hemophilia
Chromosomal Disorder
Down syndrome Turner's syndrome
Pattern of Inheritance
Autosomal recessive Autosomal dominant Sex-linked recessive
Pattern of Inheritance
47 chromosomes due to trisomy 21 XO 45 chromosomes due to a missing sex chromosome
Nondisjunction
Aneuploidy: Abnormal number of chromosomes Triploid: Extra set of chromosomes (3n) Tetraploid: Organism with (4n) Polyploid: Organism with extra sets of chromosomes - Common in plants - Results in abnormally large size plants - Some cases: responsible for new species E.g Triploid: Extra set of chromosomes (3n) Strawberries (8n) Nondisjunction: an error that sometimes occurs during meiosis in which homologous chromosome fail to separate as they should
Genomic Imprinting + Extranuclear Genes
- Two inheritance patterns that are exceptions to Mendelian inheritance - genomic imprinting + extranuclear genes Genomic imprinting: a variation in phenotype depending on whether a trait is inherited from mother or father.
- Occurs during gamete formation - caused by silencing of particular allele by methylation of DNA, therefore zygote expresses only one allele of imprinting gene - Imprint carried to all body cells and passed through generations - Imprinted gene located on autosomes Extranuclear gene are located on mitochondria and chloroplasts - DNA is small, circular, carry small number of genes - Linked to rare/severe inherited genes in humans - Mutations in these genes cause weakness/deterioration in muscles - Mitochondrial DNA is inherited only from mother because father’s mitochondria does not enter egg during formation.