Announcements ● Tutoring Center SCI I, 407 M 12-3, 5:30-6:30; W 8-9, 5:30-6:30, Th 8-12, 6-7; F 8-9 ● MasteringBiology Assignment due Thursday 5/5 LM Phases of Mitosis (b) (a) (c) (d) A. Interphase B. Prophase B. C. Metaphase D. Anaphase E. Telophase Review of Terms • A somatic cell Is a typical body cell Has 46 chromosomes in a human • A gametic cell (gamete) Is used for sexual reproduction Usually termed eggs and sperm • Haploid – cell or organism that contains one complete set of chromosomes • Diploid - cell or organism that contains two complete sets of chromosomes Review of Terms • Autosome Chromosome that is not directly involved in determining the sex of an organism • Sex chromosome A chromosome that determines whether an individual is male or female Male 44 XY Female Somatic cells 44 XX Meiosis Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis. Meiosis Chromosomes duplicate. Homologous chromosomes separate. Sister chromatids separate. Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes INTERPHASE BEFORE MEIOSIS Sister chromatids MEIOSIS I MEIOSIS II Crossing over Prophase I of meiosis Duplicated pair of homologous chromosomes In crossing over: •Homologous chromosomes exchange genetic information Homologous chromatids exchange corresponding segments. •Genetic recombination, Metaphase I the production of gene Sister chromatids combinations different from remain joined at their centromeres. those carried by parental Metaphase II chromosomes, occurs Chiasma, site of crossing over Spindle microtubule Gametes Recombinant chromosomes combine genetic information from different parents. Recombinant chromosomes Meiosis How do we account for genetic variation? Cross over: *Independent assortment *Crossing over *Random fertilization Independent Assortment: Mitosis and Meiosis Errors in Meiosis NONDISJUNCTION IN MEIOSIS II NONDISJUNCTION IN MEIOSIS I Meiosis I Nondisjunction: Pair of homologous chromosomes fails to separate. Meiosis II Nondisjunction: Pair of sister chromatids fails to separate. Gametes n1 n1 n–1 Abnormal gametes Number of n – 1 chromosomes n1 n–1 Abnormal gametes n n Normal gametes Errors in Meiosis Down Syndrome: •Is also called trisomy 21 •Is a condition in which an individual has an extra chromosome 21 •Affects about one out of every 700 children Genetics Genetics is the scientific study of heredity. Heredity is the transmission of traits from one generation to the next. Genetics Gregor Mendel •Worked in the 1860s •Was the first person to analyze patterns of inheritance •Deduced the fundamental principles of genetics Genetics Removed stamens from purple flower. Mendel studied garden peas because they: White Stamens •Easy to grow Parents (P) Carpel • Come in many readily distinguishable varieties • Easily • Can Pollinated carpel matured into pod. manipulated self-fertilize Transferred pollen from stamens of white flower Purple to carpel of purple flower. Planted seeds from pod. Offspring (F1) Genetics • A character is a heritable feature that varies among individuals. • A trait is a variant of a character. • Each of the characters Mendel studied occurred in two distinct forms. Genetics P Generation (true-breading parents) Purple flowers F1 Generation White flowers All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 Generation 3 4 of plants have purple flowers 1 4 of plants have white flowers Genetics *genes and alleles Mendel’s hypotheses (to explain his results) 1. Alternative versions of genes (alleles) account for variation in inherited characters. 2. For each character, an organism inherits two alleles, one from each parent. Genetics 3. If two alleles differ, one is dominant, the other recessive 4. The two alleles for each character segregate (separate) during gamete production. P: X DD Tall dd Dwarf F1 – all Tall Tall Dd Mendel’s Law of Segregation Genetics A Punnett Square predicts the results of a genetic cross between individuals of known genotype Tall P: DD Gamete formation: D D D D d Dwarf X dd d d d d D *genotype d 4/4 are Dd 4/4 are Tall D D d D d *phenotype *Homozygous *Heterozygous Genetics Dihybrid cross- A genetic cross between two individuals involving two characters Example: P1 GW X yellow, round green, wrinkled GGWW ggww gw gw gw F1 All yellow, round GgWw gw GW GW GW GW F1 F1 X Gw gW gw GW Gw All yellow, round All yellow, round GgWw GgWw F2 9/16 yellow, round gW gw 9:3:3:1 Phenotypic ratio; Genotypic ratio as follows: 1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw 3/16 yellow, wrinkled 1/16 GGww, 2/16 Ggww 3/16 green, round 1/16 ggWw, 2/16 ggWw 1/16 green, wrinkled 1/16 ggww Genetics Mendelian inheritance is based on probability F1 Genotypes Example- coin toss Bb female *1/2 chance landing heads Bb male Formation of sperm Formation of eggs F2 Genotypes *Each toss is an independent event *Coin toss, just like the distribution of alleles into gametes 1 2 Female gametes *The rule of multiplication – determines the chance that two or more independent events will occur together ½x½=¼ Male gametes 1 2 1 2 B B b 1 2 B B 1 4 ( 12 12 B b 1 4 b B b 1 4 ) b b 1 4 Genetics: Pedigrees First generation (grandparents) Ff Second generation (parents, aunts, and FF ff uncles) or Ff Third generation (brother and sister) Female Male Attached Free ff Ff ff Ff Ff ff ff FF or Ff Ff Human Disorders Variations in Mendel’s Laws In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents. P Generation White rr Red RR Gametes R r F1 Generation Pink Rr 1 Gametes 1 R 2 2 r F2 Generation Sperm 1 1 R 2 r 2 1 2R Eggs RR Rr 1 r Rr rr 2 Variations in Mendel’s Laws Hypercholesterolemia PHENOTYPE GENOTYPE •Dangerously high levels of cholesterol in the blood. •Is a human trait that is incompletely dominant. •Heterozygotes have blood cholesterol levels about 2X normal. •Homozygotes have blood cholesterol levels about 5X normal. HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors LDL LDL receptor Cell Normal Mild disease Severe disease Variations in Mendel’s Laws Multiple Alleles Blood Group Genotypes Red Blood Cells (Phenotype) Carbohydrate A IAIA A or IAi B IBIB or IBi AB IAIB O ii Carbohydrate B Variations in Mendel’s Laws – Pleiotropy is the impact of a single gene on more than one character. Single gene Pleiotropy Multiple traits (e.g., sickle-cell disease) Variations in Mendel’s Laws Variations in Mendel’s Laws – Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Polygenic inheritance Multiple genes Single trait (e.g., skin color) Variations in Mendel’s Laws P Generation aabbcc AABBCC (very light) (very dark) F1 Generation F2 Generation 1 8 1 8 1 8 1 8 1 Eggs 8 1 8 1 8 1 8 1 8 1 64 AaBbCc AaBbCc Sperm 1 1 1 1 1 1 8 8 8 8 8 8 1 8 20 64 1 64 6 64 15 64 15 64 6 64 Sex Linkage *1909 Thomas Hunt Morgan II III XY IV or XX *Sex chromosomes *Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male Sex Linkage Any gene located on a sex chromosome is called a sex-linked gene. • Most sex-linked genes are found on the X chromosome.