Genetics Genetics is the branch of biology that
Genetics �Genetics is the branch of biology that deals with heredity. �Research on genetics began with Gregor Mendel, who experimented with pea plants in the 1800 s.
Gregor Mendel � Mendel’s experiment- crossed pea plants that had different traits: tall x short, purple x white, round x wrinkled seeds �P (Parental Generation) True breeding plants F 1 (First Filial) offspring of the P generation --> displayed a single trait, the dominant one F 2 (Second Filial) offspring of F 1 generation
Vocabulary Terms to Know � Gene – segment of DNA that codes for a protein � � Homozygous - having two of the same allele (AA or aa); truebreeding or purebreed Heterozygous - two different alleles (Aa); hybrid Dominant - Allele that is expressed Recessive - Allele that is hidden • Dominant alleles are capital letters, recessive by lowercase - use same letter � Allele - alternate form of a gene � � Genotype - the organisms alleles Phenotype - the physical appearance
Mendel’s Laws � The Law of Dominance: one trait is covered up by another trait • A punnett square shows how gametes separate & come together during fertilization. (Locus - spot on the chromosome where an allele (gene) is located. ) � The Law of Segregation: two alleles separate during gamete formation � The Law of Independent Assortment: each pair of allele segregates independently of other pairs of alleles
PRACTICE: B=Black b=Brown Genotypes? Phenotypes?
Example EOCT Question: In humans, a widow’s peak is dominant over a continuous hairline. Mary’s father has a widow’s peak, but Mary and her mother have a continuous hairline. What is the genotype of Mary’s father? A HH B Hh C hh D cannot be determined
Example EOCT Question: Earlobe shape is a human trait. Free earlobes are dominant while attached earlobes are recessive. Two parents with free earlobes have four children. Three children have free earlobes and one child has attached earlobes. If these parents have another child, what is the probability that the child will have attached earlobes? A 25% B 50% C 75% D 100%
REVIEW Mendel’s Principles of Inheritance Inherited traits are transmitted by genes which occur in alternate forms called alleles 1. Principle of Dominance - when 2 forms of the same gene are present the dominant allele is expressed 2. Principle of Segregation - in meiosis two alleles separate so that each gamete receives only one form of the gene 3. Principle of Independent Assortment - each trait is inherited independent of other traits (chance)
Linkage Groups � Linkage Groups - In actuality, many genes that are located on the same chromosome do not assort independently, instead, they are inherited together. The closer your loci are, the more likely you are to be inherited together.
Test Cross � Test Cross: When the genotype of a parent is unknown, the parent is crossed with a recessive individual. � For example: In rabbits, brown fur is dominant to white fur. A rabbit has brown fur, but you don’t know if the alleles are homozygous or heterozygous. � Do a test cross --- cross it with a homozygous recessive (white furred rabbit). If the brown is heterozygous, then you should see white fur in the offspring.
Patterns of Inheritance �Codominance - the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms. Both alleles are expressed in the heterozygote! �Example: Red cows crossed with white will generate roan cows. Roan cows have red with white blotches. �RR = red, WW = white, RW = red & white (roan)
Patterns of Inheritance �Incomplete dominance – a cross between organisms with two different phenotypes produces offspring with a 3 rd phenotype that is a blending of the parental traits. Neither allele is dominant. �Example: In snapdragons, flower color can be red, pink, or white. The heterozygous condition results in pink flowers. �RR = red, WW = white, RW = pink
Patterns of Inheritance �Sex-linked – Some genes are located on the X chromosome. • Females receive two alleles for these genes, but males only receive one. In humans, hemophilia and color blindness are sex-linked traits.
Patterns of Inheritance �Multiple Alleles – traits are controlled by more than two alleles. Blood type is controlled by three alleles: A, B, O (o=recessive, A & B=codominant. ) Genotype Phenotype oo (Io. Io) Type O Ao (IAIo) Type A AA (IAIA) Type A Bo (IBIO) Type B BB (IBIB) Type B AB (IAIB) Type AB Polygenic Traits – traits are controlled by groups of several genes. Example: skin/eye/hair color Lethal Genes - Some genes are lethal when both alleles are present.
Pedigrees �A pedigree is a diagram of family relationships that represents genetic relationships. Pedigrees are often used to determine the inheritance of genetic diseases or traits. Female= Male= Have offspring Have disease/disorder/trait = fully shaded Carrier = half shaded No disease/disorder/trait = not shaded
� � 13 How many boys? _________ 12 How many girls? _________ 3 How many generations? ________ 4 How many with the disorder? ______
Pedigree and Sex-Linked Trait
1. How many generations? 2. How many carriers? 3. How many affected males? 4. How many affected females? 5. Autosomal or sexlinked?
Autosomal-Dominant What can you tell by the pedigree? Dominant or recessive? Autosomal or sex-linked?
SEX-LINKED What can you tell by the pedigree? Dominant or recessive? Autosomal or sex-linked?
SEX-LINKED What can you tell by the pedigree? Dominant or recessive? Autosomal or sex-linked?
Autosomal Dominant or recessive? Autosomal or sex-linked? There are NO carriers when Dominant.
Autosomal Recessive There ARE carriers when recessive.
Dihybrid Crosses �Involves two alleles instead of one.
Dihybrid Crosses �You do not need to be able to create a dihybrid cross, but you WILL need to be able to tell the genotype and phenotype from a dihybrid cross. �Trick to remember: If two organisms who are both HETEROZYGOUS for the two traits, the genotypic ratio will be 9: 3: 3: 1
Practice with Dihybrid Crosses �Genotypes: �Phenotypes:
- Slides: 30