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Chapter 9- Patterns of Inheritance

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1 Chapter 9- Patterns of Inheritance
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 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

2 Genetics Study of heredity

3 Gregor Mendel 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

4 Pea Traits

5 Genetic Cross Vocabulary
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

6 Genetic Cross Vocabulary con’t
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

7 Alleles for a gene are at the same locus (point) on homologous chromosomes



10 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

11 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

12 Mendel’s Principle of Independent Assortment
Each pair of alleles segregates independently during gamete formation


14 Dihybrid cross- cross between parents that differ in 2 traits

15 Mendel’s principles reflect probability

16 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

17 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)

18 Chart Key

19 Pedigree Chart Can you figure out carriers and genotypes from this pedigree?


21 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

22 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

23 Ultrasound/sonogram- uses sound waves to product picture of fetus

24 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

25 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

26 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

27 Pleiotrophy impact of single gene on more than one characteristic
Ex: sickle-cell disease – abnormal hemoglobin molecules are produced

28 Polygenic inheritance
affect of 2 or more genes on a single phenotype trait Ex: human skin color, height

29 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

30 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


32 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

33 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


35 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

36 Linked Genes Don’t follow Mendel’s rules
If genes are located close together they tend to be inheritted together

37 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


39 Gene Map

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