Transcript Non-Mendelian Genetics

What are the Two Laws of Mendelian
(Classical) Genetics? What are Alleles?
• Developed by Gregor Mendel (1822-1884): studied
heredity in pea plants (mainly texture and color of
seeds); based solely on observations (no knowledge of
DNA or meiosis) – see cartoon
– Law of Segregation: there are two sets of genes for a particular trait
(one from each parent), but only one gets into gamete during
gametogenesis
– Law of Independent Assortment: during gametogenesis, a gene
that enters a gamete does so independently of those for other
traits (ex. if red hair expressed, blue eyes not necessarily
expressed)
• Alleles: different forms of same gene (found at same locus)
– Dominant allele: the form expressed in offspring (if present)
– Recessive allele: masked by dominant allele (not expressed if
dominant allele present), but can still be passed on to next
generation (by a carrier)
Figure 14.1
Table 14.1; Figure 14.2
Figure 14.4
What are Genotypes and Phenotypes?
How do we Solve Genetics Problems?
• Phenotype: description of form of physical trait an individual
exhibits (ex. trait of hair color, “red hair” is a phenotype)
• Genotype: description of individual’s condition at the genetic
level; three possible genotypes:
– Homozygous dominant (AA): both genes for trait instruct to produce
dominant phenotype
– Homozygous recessive (aa): both genes for trait instruct to produce
recessive phenotype
– Heterozygous (Aa): genetic instructions conflict; for Mendelian traits,
dominant phenotype results (recessive masked)
• Solving Genetics Problems: Mendel described traits in
P (parental) generation and F1, F2 (filial) generations
– Monohybrid Cross: single trait; parents’ genotypes crossed using
Punnett Square
– Dihybrid Cross: two traits; find results for each single trait with
Punnett Square, then multiply probabilities (ex. ¼ X ¼ = 1/16)
Figures 14.6 and 14.8
Figure 14.15a
Figure 14.15b
What are Some Modern Additions to
Mendelian Genetics?
• Polygenic traits: traits caused by multiple genes
– Variation in population often follows bell curve when frequency is
plotted against measurement of phenotype (ex. height)
• Multiple alleles: ex. blood types
– Only two alleles in any cell, but more than two in population
• Linked genes: loci typically in close proximity
• Incomplete Dominance (and Co-dominance)
– Phenotype for heterozygous genotype is a mixture (blend) of
those caused by homozygous genotypes
– Problem-solving is the same as Mendelian traits; need to
evaluate genotypes differently; any mention of three phenotypes in problem?
• Examples: color of petals in roses; Sickle-cell anemia
Figures 14.10 and 14.11
Figures 14.13 and 23.13a
How is Sex Determined? What are
Some Examples of Sex-linked Traits?
• Sex Determination: 23rd pair of human chromosomes are the sex chromosomes, others are
autosomes - see cartoon
– Females: XX; Males: XY
• Sex-linked traits: X-sex chromosome has many
genes other than those for sex determination,
but Y-sex chromosome does not  no male
carriers for sex-linked traits
– Examples: color blindness, hemophilia
– Same methods to solve problems, but must account for
sex of parents and offspring [ex. XHXh x XHY  XHXH,
XHY, XhXH (= XHXh), XhY]
Figure 15.6
Figure 15.7