Biology MCAS Review: Mendelian Genetics

Inheritance Vocabulary Each individual (sexually reproducing) receive one copy of a gene from each parent Alleles Dominant Recessive Genotype Homozygous Heterozygous Phenotype

Punnett Square The gene combinations that might result from a genetic cross can be determined by drawing a Punnett square Used to make predictions about what alleles an individual would inherit based on the parents’ genes Expected vs. Observed results Possible alleles in male gametes Possible allele combinations of the offspring Possible alleles in female gametes

Mendel’s Laws of Inheritance Law of Dominance --Some alleles are dominant Always overpowers the recessive allele when together Represented with a capital letter EXAMPLE: The purple allele is dominant so Purple flowers = P --Some alleles are recessive Always overpowered by the dominant allele when present Recessive traits are only expressed when there are two recessive alleles present Represented with a lowercase letter (letter is the same as the dominant letter) EXAMPLE: The white allele is recessive so White flowers = p

Mendel’s Laws of Inheritance Law of Segregation When gametes are formed, the alleles for each gene in an individual separate from one another during meiosis. As a result, gametes only carry only allele for each trait.

Mendel’s Laws of Inheritance Law of Independent Assortment As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another independently of genes pairs on other chromosomes So…..genes that segregate independently of one another do NOT affect the inheritance of each other during gamete formation….

Possible Chromosome Combinations 1 2 3 Possible Chromosome Combinations or Maternal and paternal chromosomes are randomly aligned or or

Inheritance Patterns 1. Complete Dominance Dominant allele completely overpowers recessive alleles when present together

Complete Dominance Monohybrid Cross Example If you cross a heterozygous plant with a homozygous recessive plant, how many of the offspring will show the recessive phenotype?

Complete Dominance Dihybrid Cross Example One organism heterozygous for both traits is crossed with another individual homozygous recessive for both traits. Phenotypic ratio of offspring?

2. Incomplete Dominance One allele is not completely dominant over the other… Heterozygote shows intermediate phenotype

3. Codominance Two alleles are dominant at the same time Both phenotypes are expressed simultaneously if both present at the same time

4. Multiple Alleles More than 2 alleles for one particular gene Multiple phenotypes can result

Polygenic Traits More than one gene (and many alleles!) interact to produce one phenotype Continuous variation A continuous range of small differences in a given trait among individuals The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in that trait

6. Sex-linked (X-linked) traits Section 14-2 Inheritance of genes found on sex chromosomes XX in females, XY in males Traits are more common in males than in females…why? Go to Section:

Figure 14-13 Colorblindness Section 14-2 Father (normal vision) Normal vision Colorblind Male Female Daughter (normal vision) Son (normal vision) Mother (carrier) Daughter (carrier) Son (colorblind) Go to Section:

Pedigree: Tool used to track the inheritance of a trait through generations A square represents a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and female represents a marriage. A half-shaded circle or square indicates that a person is a carrier of the trait. A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait. A completely shaded circle or square indicates that a person expresses the trait. Go to Section:

This pedigree shows the inheritance of attached ear lobes This pedigree shows the inheritance of attached ear lobes. Which parent has attached ear lobes? 2. How many children do the parents have? Which child has attached ear lobes? Which child is married? Does this child’s spouse have attached ear lobes? Do any of this child’s children have attached ear lobes? Free earlobes are dominant over attached earlobes. Determine the genotypes of the individuals below (to the best of your ability based on the information provided to you in the pedigree).