Genetics Key Terms Terms Allele different form of

Genetics

Key Terms

Terms • Allele: different form of the same gene • Dominant: trait that is always expressed, written with a capital letter “A” • Recessive: trait that is only expressed when a dominant trait is not present, written as a lower case letter “a”

Terms • Homozygous: both alleles are the same AA or aa • Also known as a purebred • Heterozygous: the alleles are different Aa • Also known as a hybrid

• Genotype: combination of dominant and recessive alleles (Tt) • Phenotype: what you look like on the outside (tall) • Test Cross: a cross between a dominant individual of unknown genotype with a recessive individual to determine the genotype of the dominant individual

• P Generation: the 2 individuals that are used as parents in the original cross • F 1 Generation: The offspring of the P generation • F 2 Generation: The offspring of two plants crossed from the F 1 generation

Mendelian Genetics

Mendel • Carried out the first quantitative studies of inheritance. • Used garden peas. Reasons: • Can easily produce hybrid peas-knew he could observe segregation of traits. • A large number of true-breeding varieties were available. • Pea plants small and easy to grow with a relatively short generation time. • Pea plants contain both male and female reproductive organs-easy to control self or cross pollination.

Mendel's Experimental Design: • Usually contained 3 stages: • Allowed pea plants of a given variety to produce progeny for several generations through self-fertilization allowing him to be assured that some traits were pure breeding. • Performed crosses b/t varieties, exhibiting alternative forms. To do this he has to remove male parts of the flower. • Mendel permitted hybrid offspring from cross pollination to self-fertilize for several generations and counted the number of each generation exhibiting each trait.

7 Characteristics • • Dominant Form Purple flowers Yellow seeds Round seeds Green pods Inflated pods Axial Flowers Tall plants Recessive Form White Flowers Green seeds Wrinkled seeds Yellow pods Constricted pods Terminal Flowers Dwarf plants

Ratios • Mendel discovered a 1: 2: 1 ratio • ¼ pure breeding dominant (ex BB) • ½ not pure-breeding dominant (ex. Bb) • ¼ pure-breeding recessive (ex bb)

Mendel's Model of Genetics: • Parents do not transmit physiological traits, but instead factors that later act to produce these traits in offspring. • Each individual receives 2 factors that may code for the same version of a trait or a different one.

Mendel's Model of Genetics: • Not all factors are identical. • The 2 alleles do not influence each other in any way. • The presence of an allele does not ensure its expression.

Mendel’s Laws • Law of Dominance: Dominant traits will always be expressed and will mask the expression of recessive traits.

Mendel’s Laws • Law of Segregation: Each parent can only pass on one gene for each trait. Half of the sperm or eggs created will get one of each of that parents available genes.

Mendel’s Laws • Law of Independent Assortment: Different traits segregate independently and remain distinct (ex. brown hair and brown eyes are not a "package" deal. )

How do dominant traits mask recessive traits? • The dominant allele codes for a product where the recessive allele does not. Ex. Albinism is caused by a homozygous recessive gene that lacks the necessary enzyme to produce melanin. Having one dominant gene allows a person to produce enough melanin to appear normal.

How do dominant traits mask recessive traits? • The recessive allele produces less of a product so it is masked by the dominant allele that makes more product. • The recessive allele produces a fully functional enzyme that is masked by a dominant allele.

Non-Mendelian Genetics

Other dominant relationships: • Lethal recessive-homozygous recessive organisms do not have the same life expectancy as a dominant individual. (ex. tay sachs and cystic fibrosis)

Other dominant relationships: • Incomplete dominance-When the heterozygous individuals will be intermediate to the two homozygotes. • Ex. Snapdragons-RR(red)XWW(white) =RW=pink • Palomino horses (light gold color) are heterozgotes produced from this inhertiance pattern.

Other dominant relationships: • Codominance-When the heterozygote shows both homozygous traits. Ex. black rooster + white hen=black and white bird

Other dominant relationships: • Multiple Alleles-When there are more than two alleles for a trait, such as blood type. IA and IB are both dominant. • IAIA or IAi=A blood • IBIB or IBi=B blood • IAIB=AB blood • ii=O blood

Gene interactions and other modified Mendelian ratios: • Epistasis-When one gene interferes with the expression of another gene in a dihybrid cross. • Ex. Labrador coat color • eebb=yellow fur; brown nose, lips and eye rims (Yellow lab) • ee. B_=Yellow fur; black nose, lips and eye rims (Yellow lab) • E_bb=Brown fur; nose, lips and eye rim (chocolate lab) • E_B_=Black fur; nose, lips, eye rims (black)

Gene interactions and other modified Mendelian ratios: • Pleiotropy-When one gene can affect more than one characteristic. Ex. Cats with white fur also usually have blue eyes and are deaf.

Nature vs. Nurture • Your phenotype is determined not just by your genes, but also by your environment. • Ex. Nutrition influences height, exercise effects build, tanning effect pigmentation.

Nature vs Nurture • Some alleles are heat sensitve. Arctic foxes make fur pigments only when the weather is warm (white in winter, reddish brown in summer). Himalayan rabbits and siamese cats have darker fur on cooler regions of their body such as ears and tails. This is due to an enzyme that is only triggered at 33 degrees Celsius.

Gene interactions and other modified Mendelian ratios: • Incomplete Penetrance -Occurs when a person has an abnormal genotype without showing it. Ex. polydactly-a dominant trait where people have more than 5 digits on a hand or foot. In some people they may have the gene with no extra digits, or only have it on one foot, hand etc. When they have children the offspring do have a higher chance of expressing the trait.

Sex-Linked Traits • In 1910 Thomas Hunt Morgan began working with fruit flies and was the first to determine linkage of traits. • He did this by realizing that there was a higher incidence of white eyed males than females determining that the gene for white eyes was linked to the X chromosome. Since the only chromosome that does not have a homologous chromosome in the body is the male sex chromosomes he was able to trace the cause. The X chromosome carries thousands of genes where the Y only carries a few.

Human Sex-Linked Traits • The Y chromosomes carries the gene SRY which triggers the development of male reproductive structures. The X chromosome carries the DAX gene that makes female reproductive structures. • Several recessive traits are also carried on the X chromosome; many that may cause abnormalities.

Human Sex-Linked Traits • Colorblindness-Most common type is red/green colorblindness. There are 3 types of cones in the eye red, green and blue. A person who is colorblind generally lacks 1 of the 3 cones. 10% of all males are colorblind and only 0. 4% of females. To be affected a man only needs to receive one recessive gene from his mother since the Y chromosome does not carry the same genes therefore cannot mask it. Affected women must receive the recessive gene from both parents.

Human Sex-Linked Traits • Hemophilia: a recessive mutation that prevents the blood from clotting properly. It is caused by a traits the results in a missing clotting protein.

Human Sex-Linked Traits • Duchenne's muscular dystrophy-includes progressive muscle weakening, loss of coordination and hemophilia. 1 of every 3500 males in the US is affected.

Gene interactions and other modified Mendelian ratios: • Sex-limited and sex-influenced traits-A trait that is limited to or affects one gender more often than the other. Ex. a dominant gene causes a rare type of uterine cancer would not affect men, a dominant trait for baldness doesn't affect women.

Gene interactions and other modified Mendelian ratios: • Variable age of onset-Some traits do not appear till later in life. • Ex. Muscular dystrophy has different onset ages, even for related individuals. • Huntington's disease, a dominant condition, does not usually appear till after the age of 40.

X inactivation in females • Females are XX but in each cell one X chromosome is mostly inactivated and is a barr body. Small regions of the chromosome remain active but most of the genes are not expressed. Barr bodies are "reactivated" in the cells that undergo meiosis to form gametes.

X inactivation in females • The selection of which X chromosome occurs randomly and independently in each of the embryonic cells present at the time of X inactivation. Therefore the female consists of a mosaic of 2 types of cells, those with X derived from mother and those with X derived from father.

• Polygenic Inheritance: multiple genes controlling 1 trait. This results in multiple phenotypes. • Ex: skin, eye, hair color; height; weight to a degree

Alterations to the Chromosomes • Nondisjunction-Failure of chromosomes to seperate properly. Results in an individual with too many or too few chromosomes. Ex. Down's syndrome, Klinefelter's and Turner's syndrome

Alterations to the Chromosomes • Polyploidy-More than two complete chromosome sets such as triploidy=3 n or tetraploidy=4 n. This is common in plant kingdom. • Alterations of chromosome structure-The breakage of a chromosome. Ex. Cri du chat and Down's syndrome.

Alterations to the Chromosomes • Genomic Imprinting-Geneticists have identified traits that seem to depend on which parent passed along the allele. • Ex. 2 conditions caused by deletion of a segment of chromosome 15.

Alterations to the Chromosomes • Prader-Willi occurs when chromosome comes from father. Symptoms: mental retardation, obesity, short height, small hands and feet. • Angelman syndrome occurs when chromosome comes from mother. Symptoms: spontaneous laughter, jerky movements, and other motor and mental symptoms.

Prenatal Detection of Disorders • Amniocentesis-tested between 14 th and 16 th week-uses a needle to withdraw amniotic fluid and then fetal cells are grown and used to prepare a karyotype. • CVS-tested b/t 8 th and 10 th week-Chorionic villus sampling, removes a small sample of chorion (outermost membrane surrounding fetus) and fetal cells are examined for abnormalities.

Prenatal Detection of Disorders • Human genetic anomalies can be caused in the following ways: • Autosomal dominant-one gene causes the anomaly (polydactyly) • Autosomal recessive-2 genes cause the anomaly (PKU, sickle cell anemia) • X-linked recessive-found on the X chromosome (color blindness, muscular dystrophy) • Chromosomal Anomalies-trisomy 21, turner's syndrome • Multifactorial-combinations of genes and environment, both factors must be present (cleft lip, spina bifida)

Prenatal Detection of Disorders • Ultrasound-throughout pregnancy-uses soundwaves to get an image of the baby. • Newborn screening-some genetic anomalies are detected at birth.

Pedigree • Method used to track a trait through a family. • Patterns exist that allow you to identify what type of trait you are tracking (EX: dominant, recessive, sex-linked, etc) • Circles are female, Squares are male • Filled in means that they have the trait being tracked. • Half shaded means that they are carriers, not applicable to dominant traits • Sometimes carriers are not shaded in because they technically don’t express the trait being tracked

Pedigrees • Dominant traits: roughly equal numbers of men and women affected and there are no generations that are skipped.

Pedigrees • Recessive traits: roughly equal numbers of men and women affected and there are generations that are skipped.

Pedigrees • Sex-linked traits: strongly slanted to having more of one gender affected.

Karyotypes • Picture of the chromosomes in an individual • Homologous chromosomes are paired up • Arranged by size. The longest chromosome pair is chromosome 1. • The last pair is the gender chromosomes. • Used to identify nondisjunction disorders.

Nondisjunction Disorders • Turner’s Syndrome: only 1 X chromosome • Klinefelter’s Syndrome: XXY • Downs Syndrome: 3 chromosomes at the 21 st pair • Edward’s Syndrome: 3 chromosomes at the 18 th pair • Patau’s Syndrome: 3 chromosomes at the 13 th pair