Genetics

Gregor Mendel Father of modern genetics Studied pea plants variety of distinct heritable features, or characters character variations are called traits Mating of plants can be controlled used varieties that were “true-breeding” plants that produce offspring of the same variety when they self-pollinate When cross pollination occurs offspring showed a mix of characteristics of both parents

LE 14-2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Stamens Carpel Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1)

Terms Trait Gene Allele Example specific characteristic that changes from one individual to another Gene chemical factors that determine traits Found in DNA Genes for various traits are found on chromosomes Allele different forms of a gene Example Trait = plant height Alleles (different forms of gene) Short Tall

Dominance Principle of Dominance some alleles are dominant and others are recessive Dominant alleles are generally expressed using a capital letter Recessive alleles are usually expressed using a lower case letter Ex. Plant height Dominant = tall (T) Recessive = short (t) Each gene is coded for using 2 alleles One allele from each parent Dominant alleles control the appearance of the trait If two recessive alleles are present the recessive condition will be expressed

Alleles Homozygous Heterozygous individual with 2 of the same allele for a given trait Ex. TT, RR = homozygous dominant Ex. tt,, rr = homozygous recessive Can “true breed” = create offspring like self in self polination Heterozygous individual with 2 different alleles for a given trait Ex. Tt, Rr, Ww Are not “true breeders”

Genotype Phenotype The combination of alleles for a given trait Ex. TT, Tt, tt Phenotype The physical appearance caused by the interaction of alleles In general, if a dominant allele is present then the dominant condition will be shown Ex. Tall, Short

Parent F1 (first filial) F2 (second filial) First generation studied Offspring of 2 individuals from the parent generation F2 (second filial) Offspring of 2 individuals from the first filial generation

The Law of Segregation Each individual contains 2 alleles for each gene (one from each parent) During meiosis, when the number of chromosomes are reduced (diploid to haploid), gametes are produced that contain one allele for each trait Segregation = the separation of alleles for a certain trait Each gamete only carries a single allele for each gene During fertilization, two gametes fuse forming a diploid cell. This cell contains 2 alleles for each gene

(true-breeding parents) White flowers LE 14-3 P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

LE 14-5_2 3 : 1 P Generation Appearance: Purple flowers PP White Genetic makeup: Gametes P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 2 P 1 2 p F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

Phenotype Genotype PP (homozygous Purple 1 Pp (heterozygous 3 Purple 2 White 1 Ratio 3:1 Ratio 1:2:1

Probability The likelihood that a particular event will occur. Ex. If you flip a coin you have a probability of ½ that you will end up with heads. What is the probability of flipping 3 heads in a row? ½ x ½ x ½ = 1/8 The principles of probability can be used to predict outcomes of genetic crosses. Punnet Squares

Tt X Tt Cross Section 11-2 Go to Section:

Tt X Tt Cross Section 11-2 Go to Section:

The Testcross breeding the mystery individual with a homozygous recessive individual

LE 14-7 Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 2 offspring purple and 1 2 offspring white: p p p p P P Pp Pp Pp Pp P P Pp Pp pp pp

The Law of Independent Assortment Genes for different traits can segregate independently during the formation of gametes. Genes for different traits are not necessarily linked.

LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm 1 YR 1 Yr yR yr Sperm 4 4 1 4 1 4 Eggs 1 YR yr 2 1 2 1 YR Eggs 4 YYRR YYRr YyRR YyRr 1 YR F2 Generation (predicted offspring) 2 YYRR YyRr 1 Yr 4 YYRr YYrr YyRr Yyrr 1 yr 2 YyRr yyrr 1 yR 4 YyRR YyRr yyRR yyRr 3 4 1 4 1 yr 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 3 16 Phenotypic ratio 9:3:3:1

The Spectrum of Dominance Complete dominance phenotypes of the heterozygote and dominant homozygote are identical Codominance two dominant alleles affect the phenotype in separate, distinguishable ways Incomplete Dominance phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

LE 14-10 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation Gametes 1 1 2 CR 2 CW Sperm 1 2 CR 1 2 CW Eggs F2 Generation 1 CR 2 CRCR CRCW 1 2 CW CRCW CWCW

Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles Polydactyl individuals 1/400 in the U.S.

Multiple Alleles more than two allelic forms Example – blood type four phenotypes: A, B, AB, O Three alleles: IA, IB, and i.

Pleiotropy multiple phenotypic effects Ex. multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Polygenic Traits Traits controlled by 2 or more genes Ex. Eye color, hair color, skin color

Skin color 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc Fraction of progeny 6/64 1/64

Epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus Ex. Coat color of mice One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for pigment color and c for no pigment color ) determines whether the pigment will be deposited in the hair

LE 14-11 BbCc BbCc Sperm BC bC Bc bc BC BBCC BbCC BBCc BbCc bC BbCC 9 3 4 16 16 16

Nature and Nurture: The Environmental Impact on Phenotype phenotype for a character depends on environment as well as genotype Ex. hydrangea flowers same genotype range from blue-violet to pink depends on soil acidity

Human Inheritance Humans are not good subjects for genetic research generation time is too long parents produce relatively few offspring breeding experiments are unacceptable Personal Pedigree

Pedigree Analysis Pedigree family tree that describes the interrelationships of parents and children across generations Can be used to make predictions about future offspring

Dominant trait (widow’s peak) LE 14-14a First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)

Recessive trait (attached earlobe) LE 14-14b First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Attached earlobe Free earlobe Recessive trait (attached earlobe)

Down’s Syndrome Occurs in 1/800-1,000 births Caused by nondisjunction Trisomy 21 = three copies of chromosome 21

Frequency of Down Syndrome Per Maternal Age   Age (years) Frequency of Fetuses with Down Syndrome to Normal Fetuses at 16 weeks of pregnancy Frequency of Live Births of Babies with Down Syndrome to Normal Births 15 - 19 ---- 1 / 1250 20 - 24 1 / 1400 25 - 29 1 / 1100 30 - 31 1 / 900 32 1 / 750 33 1 / 420 1 / 625 34 1 / 325 1 / 500 35 1 / 250 1 / 350 36 1 / 200 1 / 275 37 1 / 150 1 / 225 38 1 / 120 1 / 175 39 1 / 100 1 / 140 40 1 / 75 41 1 / 60 1 / 85 42 1 / 45 1 / 65 43 1 / 35 1 / 50 44 1 / 30 1 / 40 45 and older 1 / 20 1 / 25 Return to Prenatal Testing for Down Syndrome Return to Down Syndrome: Health Issues Homepage

Down’s Syndrome The image shows a karyotype of a person with Down’s Syndrome, Trisomy 21

Sex Chromosome Disorders Turner’s Syndrome (XO) underdeveloped ovaries, short stature, webbed neck, and broad chest. Individuals are sterile, and lack expected secondary sexual characteristics. Mental retardation typically not evident.

Sex Chromosome Disorders Klinefelter’s Syndrome (XXY) Nondisjunction in males Some development of breast tissue, little body hair is present; typically tall, with or without evidence of mental retardation. Males with XXXY, XXXXY, and XXXXXY karyotypes have a more severe presentation, and mental retardation is expected.

Recessively Inherited Disorders Only expressed in individuals that are homozygous recessive Carriers heterozygous individuals carry the recessive allele phenotypically normal

Albinism

Cystic Fibrosis most common lethal genetic disease in the 1/2,500 people of European descent results in defective or absent chloride transport channels in plasma membranes Symptoms: mucus buildup in some internal organs abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease 1/400 African-Americans Incompletely recessive caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms: physical weakness Pain organ damage even paralysis

Sickle Cell Anemia

Dominantly Inherited Disorders Achondroplasia form of dwarfism lethal when homozygous for the dominant allele

                           Achondroplasia

no obvious phenotypic effects until about 35 to 40 years of age Huntington’s disease degenerative disease of the nervous system no obvious phenotypic effects until about 35 to 40 years of age

Some Autosomal Disorders in Humans Type of Disorder Disorder Major Symptoms Disorders caused by recessive alleles Albinism Lack of pigment in hair, skin, and eyes Cystic Fibrosis Excess mucus in lungs, digestive tract, liver; increased susceptibility to infections; death in childhood unless treated Phenylketonuria Accumulation in brain cells; lack of normal pigment; mental retardation Tay-Sachs Disease Lipid accumulation in brain cells; mental deficiency; blindness; death in early childhood Disorders Caused by dominant alleles Achondroplasia Dwarfism (one form) Huntington’s Disease Mental deterioration and uncontrolled movements; appears in middle age Disorders caused by codominant alleles Sickle Cell Anemia Sickled red blood cells; damage to many tissues

Multifactorial Disorders Genetic factors Environmental factors Ex. Cancer, heart disease

Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

Counseling Based on Mendelian Genetics and Probability Rules Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders

Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

Fetal Testing Amniocentesis chorionic villus sampling (CVS) liquid that bathes the fetus is removed and tested chorionic villus sampling (CVS) sample of the placenta is removed and tested ultrasound and fetoscopy allow fetal health to be assessed visually in utero

LE 14-17a Amniocentesis Amniotic fluid withdrawn A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. Fetus Centrifugation Placenta Uterus Cervix Fluid Fetal cells Biochemical tests can be performed immediately on the amniotic fluid or later on the cultured cells. Biochemical tests Several weeks Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Karyotyping

LE 14-17b Chorionic villus sampling (CVS) A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Fetus Suction tube inserted through cervix Placenta Chorionic villi Fetal cells Biochemical tests Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Several hours Karyotyping

Newborn Screening Tested at birth Ex. Phenylketonuria (PKU)