Semester 2- Unit 2: Inheritance
heredity -characteristics passed from parent to offspring genetics -the scientific study of heredity trait - a specific characteristic of an individual genes -factors passed from parent to offspring alleles- different forms of a gene Ex: gene: plant height alleles: tall (T) and short (t)
Gregor Mendel Mendel had a collection of true-breeding pea plants. Genetics was founded by an Austrian monk named Gregor Mendel. Worked with garden peas True-breeding plants: Self-pollinating plants that produce offspring that are identical to themselves.
principle of dominance- some alleles are dominant; others are recessive Dominant allele- capital letter (T) Recessive allele- lowercase same letter (t) organism with at least 1 dominant allele for a trait will show that trait. organism with a recessive allele for a trait will only show that form when dominant allele is not present.
P generation- original pair of plants in a cross (parents) F 1 generation- offspring of the P cross. F 2 generation- offspring from an F 1 cross
homozygous- having 2 identical alleles for a particular gene (ex: TT or tt ) heterozygous- having 2 different alleles for the same gene (ex: Tt or Rr or Gg) phenotype- physical traits; what is seen; a description (ex: tall, round, green) Genotype- genetic makeup, 2 letter code; allele combination (ex: TT, Rr, gg)
Punnett square- used to predict the outcome of a genetic cross by drawing a simple box table diagram They predict the genotype & phenotype combinations in genetic crosses using probability
Probability- likelihood an event will occur - Ex: probability of flipping a coin & getting heads is 50% or ½ - Flipping a coin 3 times in a row & getting heads is ½ ½ ½ = ⅛ (rule of multiplication) - Each event is independent of the previous one Principles of probability can be used to predict the outcomes of genetic crosses.
principle of independent assortment- genes for different traits segregate independently during the formation of gametes. dihybrid cross- involves 2 different genes or 2 factors in a cross. (punnett squares can be 1 box to 16 boxes big) monohybrid cross- single-gene or 1 factor crosses. (4 box punnett squares)
incomplete dominance- one allele is not completely dominant over another the heterozygous phenotype is between the two homozygous phenotypes (blending of alleles) Ex: Red (RR) crossed with White (WW) produces Pink (RW)- a blend of the 2 alleles
codominance- the phenotypes produced by both alleles are clearly expressed (without blending). Ex: certain varieties of chicken, allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have speckled black & white feathers.
multiple alleles- a gene with more than 2 alleles Ex: the alleles for the genes for human blood type a gene with 3 alleles: I A, I B, and i. I A & I B are codominant; i is recessive *person w/ AB - (neg) blood: has I A & I B alleles from the ABO gene & 2 Rh - alleles from the Rh gene.
polygenic traits- many genes, single trait controlled by 2 or more genes show a wide range of phenotypes. Ex: *Variety of skin color in humans comes about partly because 4 different genes control this trait. *Height in humans
Environmental conditions can affect how genes are expressed & influence traits Genes provide a plan for development, but the plan also depends on the environment. Phenotype is only partly determined by its genotype Ex: temperature may affect color expression
karyotype- display showing the diploid (2n) set of chromosomes grouped together in pairs, arranged in order of decreasing size. Human Karyotype= 46 chromosomes
autosomes- first 44 chromosomes, or pairs 1-22 sex chromosomes- determine an individual s gender; last 2 of the 46 chromosomes *Females: XX is the 23 rd pair *Males: XY is the 23 rd pair Males & females- born a roughly 50 : 50 ratio. All human egg cells carry a single X chromosome (23,X) Half of all sperm cells carry an X chromosome (23,X) & half carry a Y chromosome (23,Y).
Sex Chromosomes: More than 1200 genes found on the X chromosome Y chromosome smaller than X; contains about 140 genes- mostly with male sex determination & sperm development.
sex-linked traits: Due to genes located on the X or Y chromosomes Genes on the Y chromosome- found only in males & are passed from father to son. Genes located on the X chromosome- in both sexes Since males have just 1 X chromosome- Males show dominant & recessive traits on the X chromosome since they only have 1 X Recessive phenotype of a sex-linked genetic disorder is more common in males (than females). Females need 2 recessive alleles, 1 for each X chromosome, to show the trait -Red-green color blindness & Hemophilia- recessive on X
Human Pedigrees: Pedigree- chart showing the relationships within a family to analyze the pattern of inheritance of a particular trait shows the presence or absence of a trait according to the relationships between parents, siblings, & offspring. can determine if an allele for a trait is dominant or recessive, autosomal, or sex-linked.
symbols in a pedigree: circle = female shaded symbol= trait square = male unshaded= no trait rows of symbols= # of generations (roman numerals) Individuals= normal numbers (left to right= oldest to youngest) Horizontal line= relationship Vertical line= connects offspring Carrier= person that inherits the recessive allele but does not express the trait
How many total males? Females? How many males with the trait? Females? Is the trait dominant or recessive? How many generations?