AP Biology-UNIT VI: HEREDITY Study Notes

A. Mendelian Genetics
➢ Genetics=study of heredity

•  Explains how certain characteristics are passed from parents to children
•  Heredity=transmission of traits from one generation to the next

➢ Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
➢ Physical traits are not inherited; genes are inherited
➢ Gregor Mendel=”father of genetics”
➢ Traits

•  Expressed characteristics
  ■ character=feature(ex. Eye color); trait =specific version of that feature (ex. blue eyes)
• Influenced by one or more genes
  ■ Gene=chunk of DNA that codes for a particular “recipe”
•  DNA is passed from generation to generation, and genes/traits go along with it
•  Chromosome contains many genes, each controlling the inheritance of a particular trait
•  Locus=position of a gene on a chromosome
• Children do not inherit physical traits; they inherit genes, which influence physical traits
  ■ Genes passed along by gametes (sperm/egg)

➢ clone=group of genetically identical individuals from same parent

•  common in asexual reproduction
•  Sexual reproduction creates genetic diversity

➢ Diploid organisms typically have 2 copies of a gene, one on each homologous chromosome

• Copies of chromosome may be different from each other, containing different alleles

➢ Homozygous=organism has 2 identical alleles for a given trait

➢ heterozygous=organisms has 2 different alleles for a given trait

➢ Phenotype=physical appearance

➢ Genotype=genetic makeup

➢ Dominant vs. recessive allele

•  Dominant allele is determined by which allele is the phenotype of a heterozygous organism
•  Dominant allele showed by capital letter; recessive allele showed by lowercase of same letter

➢ Crosses

• 1st generation in an experiment is always called the parent/P1 generation
• Offspring of P1 are called the filial/F1 generation
• Offspring of F1 are called F2 generation, etc.

➢ true-breeder=genetically pure; consistently produces same traits

➢ Law of Dominance

•  One dominant trait masks the effect of the other trait

➢ Law of Segregation

•  Monohybrid Cross
  ■ 2 heterozygous individuals are crossed
  ■ Ratios for cross of two heterozygotes
  ● Phenotype ratio= 3 dom.:1 rec.
  ● Genotype ratio= 1 homo dom: 2 het: 1 homo rec
•  Gametes only get one of the 2 copies of a gene

➢ Law of Independent Assortment

•  Each allele of the two traits will get segregated into two gametes independently and randomly along Metaphase plate of meiosis I
• Each pair of chromosomes sorts maternal/paternal homologues independently of the other
  pairs
  ○ For humans, (n=23), there are more than 8 million $(2^{23})$ possible combinations of
  chromosomes, not including crossing over, mutations, etc.
  ○ Dihybrid cross
  ■ 2 heterozygotes for two genes are crossed
  ■ 9:3:3:1 ratio
  ■ Easier to use probability rather than a punnett square

➢ Random Fertilization also creates genetic variability

•  Any sperm can fuse with any ovum
• 70 trillion diploid combinations

➢ Rules of Probability

• Probability of 2 independent traits occurring together= probability of trait A*probability of trait B

➢ Test Cross

• How to tell if an organism displaying dominant phenotype is homo-dom or het: USE TESTCROSS
• Breed mystery organism with a homo-rec
  ■ If all offspring display dom phenotype, the organism is homo-dom
  ■ If any offspring display rec phenotype, the organism is het

➢ Linked Genes: group of genes on same chromosome tend to stay together/inherited together

•  Cannot segregate independently since they are on the same chromosome, violating the law of independent assortment
• Can only be separated by crossing-over
•  recombinant=offspring formed from recombination events

■ Percentage of recombination=

                 ● Can be used as a measure of how far apart genes are/order
                 ● Distance on a chromosome is measured in map units aka centimorgans on a linkage map
                ● One map unit=$1%$ recombination frequency
                ● Farther apart 2 linked alleles are on a chromosome the more often the chromosome will cross over between them
                ● Genes on different chromosomes have $50% $recombination frequency
➢ Karyotype: ordered display of the pairs of chromosomes in a cell

•  2 chromosomes in a pair=homologous chromosomes

➢ PEdigrees: show family history of allele(S)

•  Describes interrelationships of parents and children across generations
•  Inheritance patterns of particular traits can be traced back and described using pedigrees

➢ Alterations of Chromosome Number/Structure

• Nondisjunction
  ■ Pairs of homologous chromosomes don’t separate properly during meiosis
  ■ One gamete receives 2 of the same type of chromosome (trisome) while the other receives none (monosome)
  ■ Results in Aneuploidy
• Deletion
  ■ Removes a chromosomal segment
  ■ CDE➝CE
•  Duplication
  ■ Repeats a segment
  ■ CDE➝CDCDE
•  Inversion
  ■ Reverses orientation of segment within a chromosome
  ■ CDE➝EDC
• Translocation
  ■ Moves a segment from one chromosome to another

➢ Genome imprinting

• Phenotype depends on which parent passed along alleles for trait
•  Involves silencing of certain genes that are “stamped” with an imprint during gamete production
•  Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg