Presentation is loading. Please wait.

Presentation is loading. Please wait.

Patterns of Inheritance

Similar presentations

Presentation on theme: "Patterns of Inheritance"— Presentation transcript:

1 Patterns of Inheritance
Chapter 9 BIOL 1010 Patterns of Inheritance

2 GENETICS: the scientific study of heredity
Genome: complete set of an organism’s gene Gene: unit of heredity which codes for a protein Siberian huskies: look alike because they share a similar genetic history Thick double coat, well-insulated ears, great stamina Inbreeding: purebreds tend to concentrate ‘bad’ genes (i.e. PRA) A dog’s behavior is determined by its genes and by the environment Nature versus nurture

3 Gregor Mendel: “Father of Modern Genetics”
Gained posthumous fame as the founder of genetics Was the first person to analyze patterns of inheritance: “heritable factors” Deduced the fundamental principles of genetics Austrian scientist and Augustinian friar Most famous for working with pea plant hybridization experiments in St. Thomas Abbey (geneticists have since ID’d the genes that controlled for traits Mendel used Could completely control fertilization, short generation time, many offspring, easily identifiable traits 1866: Published work (seven years after Darwin’s Origin of Species) Rejected at first; Darwin and other scientists unsuccessful at trying to explain inheritance Not until the 1900’s that the importance of his work was realized Research was experimentally and mathematically rigorous, has stood the test of time

4 Rules of Probability Rule of Multiplication: probability of a compound event is the product of the separate probabilities of the independent events P(A and B) = P(A)*P(B) Rule of Addition: if events are mutually exclusive, then the probability of either/or is the sum of the separate probabilities of the independent events P(A or B) = P(A) + P(B) The probability of event A AND event B occurring is the chance of event A occurring multiplied by the chance of event B occuring The probability of event A OR event B occurring is the chance of event A occurring added to the chance of event B occuring

5 Genetic Nomenclature Heredity: transmission of traits from one generation to the next Phenotype (Character): heritable feature (i.e. flower color) based on genotype (genetic makeup) Trait: variant of a character Wild-type: variant found most often in nature True-breeding: purebred, offspring are identical to the parent Hybrids: offspring of two different true-breeding parents The seven characters of pea plants studied by Mendel Wild-type: doesn’t necessarily mean dominant. (i.e. freckles, a dominant trait, is less frequent in humans than no freckles, which is recessive)

6 Inherited Traits in Humans: Controlled by a Single Gene
*Most other traits such as hair or eye color are multigenic (polygenic) traits

7 Genetic Crosses Monohybrid Cross P generation: two different pure-breeding parental plants F1 hybrids: the first generation plants obtained from crossing two selected pure breeding plants. F1 hybrids do not produce seed that is the same as the parent plants F2 hybrids: second generation plants (result of self or cross fertilization of F1 hybrids

8 Monohybrid Cross A monohybrid cross is a cross between purebred parents that differ in only one characteristic F1 generation: all show the trait of one parent (i.e. purple flowers) F2 generation: show the two traits of the parents in a 3:1 ratio (i.e. purple to white flowers) Monohybrid Cross Figure 9.5 Mendel's cross tracking one character (flower color). (Step 3)

9 Mendel’s Law of Segregation
There are alternate versions of genes For each inherited character, an organism inherits two alleles, one from each parent Homozygous: two identical alleles Heterozygous: two different alleles If the two alleles differ (the individual is heterozygous) Dominant allele: determines the phenotype (character) Recessive allele: no noticeable effect on the phenotype (character) Law of segregation: A sperm or egg carries only one allele for each character because the allele pair segregates during meiosis Based on Mendel’s observations, he came up with four hypotheses

10 Punnett Squares Show the results of random fertilization
Each axis shows possible alleles from each parent

11 Test Crosses: Determining Unknown Genotypes
Mating of an individual of dominant phenotype but unknown genotype to a homozygous recessive individual

12 Modern Genetics Modern genetics
Homologous chromosomes contain the same genes at the same loci but may contain different alleles (alternative forms of a gene) Gene locus (plural-loci): specific location of a gene on the chromosome Gene loci: BRCA1: 17q21.31 (long arm of chromosome 17, band 21, sub-band 31)

13 Dihybrid Cross A dihybrid cross is a cross between purebred parents that differ in two characteristics F1 generation: all show the dominant trait of the two characters F2 generation: show four combinations of traits in a 9:3:3:1 ratio Figure 9.5 Mendel's cross tracking one character (flower color). (Step 3)

14 Mendel’s Law of Independent Assortment
Each pair of alleles assorts independently of the other pairs of alleles during gamete formation (the inheritance of one character has no effect on the inheritance of another) Based on Mendel’s observations, he came up with four hypotheses

15 Mendel’s Laws and Meiosis
Chromosome Theory of Inheritance: genes are located at specific loci on chromosomes and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns Law of segregation: two allele pairs segregate into different gametes during meiosis Law of Independent Assortment: each pair of alleles for a particular character segregate independently of each other How can genes located on the same chromosome be assorted independently?

16 A family pedigree showing inheritance of free versus attached earlobes
Male Female Affected Male Affected Female Mating between related individuals A family pedigree showing inheritance of free versus attached earlobes

17 Some Autosomal Disorders in Humans
Recessive Disorders Albinism (1/22,000) Cystic fibrosis (1/1,800) Phenylketonuria (1/10,000) Sickle cell disease (1/500) Tay Sachs disease (1/3,500) Dominant Disorders Achondroplasia (1/25,000) Alzheimer’s disease (early onset) Huntington’s disease (1/25,000) Hypercholesterolemia (1/500) Recessive alleles (even lethal) persist indefinitely in populations in heterozygous carriers Albinism: lack of pigment in skin, hair and eyes Cystic fibrosis: excess mucus in lungs, digestive tract, liver; increased susceptibility to infections, death in early childhood unless treated -30,000 in US, 70,000 worldwide, 1/25 people are carriers Phenylketonuria: lack enzyme necessary to break down phenylalanine; accumulation of phenylalanine in blood; lack of normal skin pigment, mental retardation unless treated Sickle cell disease: sickled red blood cells; damage to many tissues Tay Sachs disease: lipid accumulation in brain cells, mental deficiency, blindness, death in childhood Achondroplasia: dwarfism Alzheimer’s disease: mental deterioration; usually strikes late in life Huntington’s disease: mental deterioration and uncontrollable movements; strikes in middle age Hypercholesterolemia: excess cholesterol in blood; heart disease

18 Inbreeding Increases the Likelihood that a Recessive Trait will be Inherited
Inbreeding: mating between close blood relatives Increases the likelihood of homozygosity of a recessive allele in children of inbred parents Most genetic disorders are not evenly distributed across all ethnic groups Result of prolonged isolation of certain populations Dd DD DD Dd DD Dd Dd Dd Early inhabitants of Martha’s Vineyard (island off the coast of Massachusetts) letd to frequent marriages between close relatives Frequency of an allele that caused deafness was high Inbreeding in dogs: many purebreds have known genetic defects Inbreeding in endangered species: not many mates to choose from European ancestry: cystic fibrosis, phenylketonuria Ashkenazi Jews: Tay Sachs disease African ancestry: sickle cell disease dd=deaf

19 Dominant Disorders One allele is all that is required to cause the phenotype i.e. Achondroplasia: dwarfism (homozygosity=lethal) Dominant lethal alleles are rare in populations i.e. Huntington’s disease: causes degeneration of the nervous system in middle age Family with and without achondroplasia: Amy Roloff and husband Matt (unrelated form of dwarfism) have four children, one with achondroplasia, three without Achondroplasia: dwarfism, head and torso develop normally, arms and legs are short Fibroblast growth factor receptor 3 which causes an abnormality of cartilage formation leading to shortened bones Why are dominant lethal alleles rare?

20 Genetic Testing Carrier screening Prenatal diagnostic testing:
i.e. amniocentesis Newborn screening Genealogical DNA testing Predicting adult-onset disorders Estimating risks of disease development Confirmational diagnosis Forensic/identity testing About 200,000 new breast cancer cases are diagnosed each year Approximately 10% of these are heritable; they run in families There are options if the results of genetic testing show a positive result for a BRCA mutation Genetic counseling: (interdisciplinary field between biology and psychology) Prenatal Diagnostic Testing Amniocentesis: physician removes ~2tsp of amniotic fluid with a needle for genetic testing Chorionic villus sampling: physician collects some placental tissue for genetic testing Newer methods: able to isolate fetal cells from mother’s blood Ashkenazi Jewish ancestry: higher risk for Tay Sachs, Fanconi anemia, cystic fibrosis and others Ethical issues?

21 Variations on Mendel’s Laws
Incomplete dominance: F1 has an appearance in between the two parental phenotypes Codominance: both alleles are fully expressed in heterozygous individuals Pleiotropy: single gene influences more than one character Polygenic Inhertitance: additive effects of two or more genes on a single phenotype Environmental Factors: non-genetic, non-hereditary factors that contribute to phenotype Mendel was lucky he chose peas and not snapdragons Environment: weight, skin color, health and fitness, etc.

22 An Example of Incomplete Dominance
Hypercholesterolemia Hypercholesterolemia: gene that codes for LDL receptor is defective, cells unable to mop up excess LDL from blood HH: normal individuals Hh: blood cholesterol levels about 2x normal, may have heart attacks by mid-30s hh: blood cholesterol levels about 5x normal, may have heart attacks as early as age 2

23 An Example of Codominance
ABO Blood Groups ABO Blood Groups: three alleles for blood type, IA, IB and i Clumping of donor blood cells can kill the recipient What blood type is the universal donor? What blood type is the universal acceptor? Blood types: Out of 100 people 38 will be O positive 7 will be O negative 34 will be A positive 6 will be A negative 8 will be B positive 2 will be B negative 4 will be AB positive 1 will be AB negative

24 An Example of Pleiotropy
Sickle-Cell Disease Sickle-cell disease: defective gene causes abnormal hemoglobin proteins Sickle cells are rapidly destroyed by the body Sickle cells don’t flow smoothly in blood and tend to clump and clog

25 An Example of Polygenic Inheritance
Skin Color Skin color: controlled by at least three genes

26 Linked Genes Thomas Hunt Morgan: In 1916, published a paper on genes in Drosophila melanogaster (fruit fly) Found the recombinant frequency to be 17%, not 50% Punnett square on expected outcomes assuming independent assortment (575) and compare with actual outcomes Calculate recombination frequency (391/2300) and convert into cM (centiMorgans) Why are recombination frequencies generally not over 50%

27 Linked Genes Genetic recombination (crossing over): usually ensures that genes on the same chromosome still assort independently Linked genes: genes so close together on a chromosome that they do not assort independently but tend to travel together Linkage map: diagrams describing relative gene locations using recombination frequencies The shorter the distance between two genes, the less likely a crossover event will happen between them

28 Sex Chromosomes and Sex-Linked Genes
Sex chromosomes: X and Y XX  Female XY  Male Sex-linked gene: any gene located on a sex chromosome X-linked genes: the X chromosome contains many more genes (~2000 genes) than the Y chromosome (~78 genes) Sex-linked disorders: disorders associated with a defective gene found on a sex (usually X) chromosome Why doesn’t the Y chromosome contain as many genes as the X chromosome? Why are none of the Y chromosome genes necessary for survival?

29 Sex-Linked Disorders in Humans
Red-green color blindness: recessive X-linked trait Is characterized by a malfunction of light-sensitive cells in the eyes. Males are more likely to be color blind than females Red-green colorblindness: individual with normal color vision can see 150+ colors; individuals with red-green colorblindness can see fewer than 25 Why are males more affected than females? Doesn’t two X chromosomes present a dosage problem? Calico cat example Figure 9.30 Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings

30 Sex-Linked Disorders in Humans
Hemophilia: “The Royal Disease” Sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises Figure 9.32 Hemophilia in the royal family of Russia. Figure 9.32

Download ppt "Patterns of Inheritance"

Similar presentations

Ads by Google