Chapter 9- Patterns of Inheritance • ABO blood groups • Alleles • Amniocentesis • Carrier • Chorionic villus sampling • Chromosome theory of inheritance • Codominance • Cross • Cross-fertilization • Cystic fibrosis • Dihybrid cross • Dominant allele • F1 generation • F2 generation • Genetics • Genotype • Hemophilia • Hermaphroditic • Heterozygous • Homozygous • Hybrids • Incomplete dominance • Linked genes • Monoecious • Monohybrid cross • P generation • Pedigree • Phenotype • Pleiotropy • Polygenic inheritance • Principle of independent assortment • Principle of segregation • Punnett square • Recessive allele • Recombination frequency • Rule of addition • Rule of multiplication • Self-fertilize • Sex chromosomes • Sex-linked genes • Testcross • Ultrasound imaging
Genetics • Study of heredity
Monk, father of modern genetics, 1860’s Discovered genetic principles by meticulous breeding of pea plants Used peas because they were readily available, easy to grow, had many distinguishable traits, could control mating Gregor Mendel
Pea Traits
Self-fertilize- sperm from pollen fertilizes egg containing carpel Cross-fertilization- fertilizing one plant from pollen of another plant Offspring produced are hybrids P generation (parent) F1 generation – offspring of P F2 generation- offspring of F1, can be self- or cross- fertilized Genetic Cross Vocabulary
Monohybrid cross- parents differ in only one trait Alleles- alternate forms of a gene A (dominant) or a (recessive) Homozygous- alleles for a trait are identical, AA Heterozygous- alleles are different, Aa Phenotype- organism’s physical trait expressed Ex: purple flower Genotype- genetic make up of trait Ex: PP Genetic Cross Vocabulary con’t
Alleles for a gene are at the same locus (point) on homologous chromosomes
Mendel’s hypotheses: • There are alternative forms of genes (units that determine traits) • For each characteristic- an organism has a gene from each parent, they can be the same allele or different • Sperm and egg each have 1 allele for a trait • Idea of dominant and recessive alleles
Mendel’s Principle of Segregation • Pairs of genes segregate (separate) during gamete formation (what is that process called?) • gametes fusing during fertilization regain pairs of genes
Mendel’s Principle of Independent Assortment • Each pair of alleles segregates independently during gamete formation
Dihybrid cross- cross between parents that differ in 2 traits
Mendel’s principles reflect probability
How can we determine an unknown genotype? • Testcross- mating between an unknown individual and a homozygous recessive individual • Rule of multiplication- probability that a compound event is the product of the separate probabilities of the independent events • Ex: b from mom (1/2), b from dad (1/2) • baby being bb ½ x ½ = ¼ • Rule of addition- probability that an event can occur in 2 or more alternative ways is the sum of the separate probabilities of different ways • Ex: in Punnet square Bb = ¼ the other Bb is ¼ • probability of being Bb is ¼ + ¼ = 2/4 = ½
What about humans? • To find human inheritance information: • Collect as much family data as possible • Construct pedigree chart • At first heterozygotes are not known, must determine phenotypes of offspring then could lead to information on carriers (heterozygotes)
Chart Key
Pedigree Chart • Can you figure out carriers and genotypes from this pedigree?
Many genetic disorders are a result of a single gene • Most are recessive • family may not know of defect until offspring with two recessive alleles is born • Ex: CF cystic fibrosis • Dominant traits are not always prevalent just because they are dominant, usually due to lethality of disorder (death before reproduction) • Ex: polydactyly, achondroplasia
How can we detect genetic disorders? • Fetal testing • Amniocentesis- cells from amnionic fluid is tested • CVS- chorionic villi sampling- fetal tissue from placenta is tested
Ultrasound/sonogram- uses sound waves to product picture of fetus
Genetic screening • Most test DNA but some test enzymes • Most tell risk of disorders that depend on multiple genes • Types of testing • Carrier- determine if you are a carrier of a harmful allele • Diagnostic- confirm or rule out disorder • Prenatal- checks for disorders in fetuses • Newborn- catches inherited disorders quickly so medical attention can be given, “heel prick” • Predictive- test at any time to determine risk for developing disorder • Ex: BRAC1 and 2- linked to breast cancer, Huntington’s disease
Variations of Mendel’s Principles Genotype/phenotype relationship is not always straightforward • Incomplete dominance • produces intermediate phenotypes • Ex: hypercholesterolemia • HH= normal, Hh=slightly affected, hh=has disease severely
Codominance • when both alleles are expressed in the phenotype • Ex: ABO bloodgroups • Some genes have more than 2 alleles, each individual can only have 2 • A, B, AB or O bloodgroups –AB is codominant
Pleiotrophy • impact of single gene on more than one characteristic • Ex: sickle-cell disease – abnormal hemoglobin molecules are produced
Polygenic inheritance • affect of 2 or more genes on a single phenotype trait • Ex: human skin color, height
Other genes are on sex chromosomes • Sex chromosomes- determine sex in species • Humans- X and Y are sex chromosomes • Each have 44 autosomes (22pairs) and 1 pair of sex chromosomes XX-female or XY- male • Males determine sex of offspring because they can pass X or Y, females only pass X • SRY gene on Y chromosome triggers testes development, without it, ovaries are developed • Fruit flies- # of X’s determine male or female, however presence of Y is essential for sperm production
Sex chromosomes con’t • X-O system- grasshoppers, crickets, roaches • Female- XX male- XO (O=absence of chromosome) • Z-W system- fish, butterflies, birds • Males-ZZ females-ZW, eggs determine sex • Determination by chromosome #- ants and bees • Females- diploid- develop from fertilized eggs • Males- haploid- develop from unfertilized eggs
Some can produce both sperm and eggs • Monoecious- plants that produce sperm and eggs • ex: corn • Hermaphroditic- animals that produce sperm and eggs • ex: earthworms
Sex- linked genes • Genes located on sex chromosomes • Not related to sex determination • Mostly found on X • In humans X-linked recessive traits mostly affect males • Females are carriers and pass to sons, sons have no other X to dominate over the recessive allele • Ex: colorblindness, hemophilia
Chromosome behavior accounts for Mendel’s principles: • Chromosomal Theory of Inheritance • Genes are located on chromosomes, the behavior of chromosomes (segregation and independent assortment- Mendel’s ideas) during meiosis and fertilization accounts for the inheritance patterns
Linked Genes • Don’t follow Mendel’s rules • If genes are located close together they tend to be inheritted together
Linked Genes con’t • Data from crossing over can lead to mapping genes, farther apart they are greater chance of crossing over in between the genes • Crossing over accounts for new gene combinations • Morgan worked with fruit flies to develop: • Recombination frequency- % of recombinants in offspring
Gene Map
