Unit 8 Meiosis and Mendel Genetics and Inheritance Quiz Date: Jan 14 Test Date: Jan. 22/23
UNIT 8 - INTRODUCTION TO GENETICS Although the resemblance between generations of organisms had been noted for thousands of years, it wasn t until the 1800s that scientific studies were carried out to develop an explanation for this. Today we know that we resemble our parents because of, heredity which is the set of characteristics we receive from. our parents The study of heredity is known as. genetics
I. SEXUAL REPRODUCTION & MEIOSIS (pp. 275-278) In sexual reproduction, an egg and sperm cell fuse together to create a fertilized egg or. zygote
A. Chromosome Number Body 1. Somatic Cells - cells Human somatic cells contain 46 chromosomes Diploid or 2n meaning they contain a double set of chromosomes, half ( 23 ) from Dad and half from. Mom Matching chromosomes known as. Homologous pairs made up of a copy of a chromosome from each parent, with the same, genes but they are. not identical mitosis Somatic cells use for growth, development, renewal, and repair
Homologous Chromosomes
GAMETES 2. Gametes - Egg and Sperm cells Human gametes contain 23 chromosomes. Haploid or n meaning there is one set of instructions for each. gene When gametes fuse together in, fertilization the zygote produced is diploid and has 46 chromosomes. Gametes are produced by a process called, meiosis rather than mitosis
Haploid vs. Diploid Karyotypes
B. Meiosis Special type of cell division that only occurs in specialized germ cells (sex cells) in ovaries of females and testes of males. Before meiosis, DNA is replicated once (during S phase of ) interphase But during meiosis cell divides, 2X, two times resulting in 4 cells with 1/2 the original chromosome number. Unlike mitosis which produces identical cells, meiosis produces genetically non-identical cells.
Identical somatic cells Unique gametes
Meiosis occurs in two stages:
1. Meiosis I Prophase I Unlike in prophase of mitosis, In prophase I Homologous pairs come together and stick to each other to form a. tetrad
Crossing Over Meiosis I Exchange of genetic information between a Sister chromatid with its. Non-sister homologue Occurs very frequently Allows for. Genetic variation
Metaphase I Meiosis I Tetrads align in equator of cell Each homologue consists of. Two sister chromatids Anaphase I Homologous pairs are pulled apart, Centromeres Sister chromatids still intact
Telophase I Meiosis I Two cells are formed, each with chromosomes Each chromosome still composed of two Sister chromatids 23 Two cells produced at the end of meiosis I are haploid There are no homologous pairs present because
MEIOSIS II
2. Meiosis II Prophase II Continues with the two cells formed moving directly into prophase II without any further replication of DNA. Metaphase II Chromosomes (not tetrads) composed of Sister chromatids are randomly aligned at the equator of the cell.
2. Meiosis II Anaphase II Spindle fibers shorten Sister chromatids are pulled apart, just like in anaphase of mitosis. Telophase II Four nuclei form around chromosomes, spindles break down
2. Meiosis II Cytokinesis II Two new cells are formed from each of the two cells formed in meiosis I, resulting in a total of 4 new cells, each with the 1/2 original number of chromosomes. Cells produced are called. Gametes
MEIOSIS In females, process is known as. oogenesis In males, process is known as. spermatogenesis
3. Oogenesis vs. spermatogenesis Spermatogenesis: Males produce 4 viable sperm cells Begins: puberty Ends: death
3. Oogenesis vs. spermatogenesis Oogenesis: Females produce only one egg and 3 polar bodies Begins: Before birth Ends: menopause
Spermatogenesis vs Oogenesis
II. HISTORY OF GENETICS (pp. 263-270) A. Gregor Mendel Known as the Father of Genetics Famous for his experiments with pea plants.
Mendel Used true-breeding pea plants, which means ; pureline, purebred characteristics always show. Known as the P generation. Studied seven, characters including plant height, seed color, flower color, etc. Inherited characteristic o A character is an. o A trait is a variant of a character o Example: Eye color is a character, brown eyes or blue eyes are traits
Mendel Pea plants cross-pollinate, meaning pollen from one plant fertilizes an egg from another, but they can also self-pollinate, meaning pollen can fertilize egg from same plant. Mendel controlled the fertilization process of the pea plants by preventing Self-pollination and controlling. Cross-pollination
Pea Traits Studied
B. Mendel s Results True-breeding P generation Crossed plants with one trait with True-breeding plants with the other. For example, TT (tall) x tt (short) F1 generation Offspring produced from. P X P In F 1, one trait. disappeared For example, tall plants X short plants =. All tall plants F2 generation Offspring produced from. F1 X F1 In F 2, trait that disappeared in F 1 reappeared in 1/4 of the offspring; the other ¾ showed. Dominant trait
Mendel s Principles C. Mendel s Principles After analyzing his results carefully, Mendel formed conclusions that increased understanding of inheritance and opened the door for the study of genetics. Individual units called genes determine inheritable characteristics. A gene is a portion of DNA that codes for a specific. trait
Alleles and Genes
Mendel s Principles For each gene, an organism inherits two alleles, one from each. parent Alleles are different forms or versions of a. gene For any given trait, homozygous o If an organism is (for a gene), its alleles are the same and the trait will be expressed. o If the alleles differ, the organism is said to be heterozygous for that character and only one allele will be expressed. The expressed allele is the dominant allele, designated by an -case upper letter. The allele that is not expressed in a heterozygous trait is, recessive designated by a -case lower letter. A recessive allele is only expressed when an organism is homozygous for that gene.
Mendel s Principles Principle of Segregation - In meiosis, the two alleles for a trait segregate ( ). separate Each egg or sperm cell receives a copy of one of the two alleles present in the somatic cells of the organism. There is a 50% chance that a copy of that allele will end up in the gamete produced. Principle of Independent Assortment The way one pair of alleles segregates has no influence on any other pair of alleles.
Independent Assortment
D. Genetics Terminology Physical 1. Phenotype - description of trait; for example, Tall, short 2. Genotype Genetic make-up of an organism or set of alleles; for example,. TT, Tt, heterozygous, etc..
Genetics Terminology 3. Application of Terminology - If round pea seeds are dominant to wrinkled pea seeds, round is designated R and wrinkled is designated. r a. Homozygous dominant Genotype = ; RR Phenotype = round b. Heterozygous Genotype = ; Rr Phenotype = round c. Homozygous recessive Genotype = ; rr Phenotype = wrinkled
III. ANALYZING INHERITANCE (pp.135-137) A. Probability Due to the law of, segregation if you know the genotype of the parents, you can predict the likelihood of a trait occurring in the offspring. Probability can be written 3 ways. The probability of a coin coming up 1/2 heads after being flipped is (fraction), (ratio), or (percent). 1:1 50%
B. Punnett Squares A Punnett square is a tool used to predict the possible outcomes of meiosis and ; fertilization in other words, a Punnett square is used to determine the probability of certain traits appearing in offspring.
Punnett Practice Monohybrid crosses A-D Must have a key And Cross with Each problem For full credit
A: Construct a Punnett square to determine the probability of white flowers is a heterozygous purple flower (Pp) is crossed with a homozygous white flower (pp) 1. Key: 2. Cross: p P=purple, p=white Pp X pp P p Pp pp Probability of White flowers: 50% p Pp pp
B: Construct a Punnett square to determine the probability of short pea plants if a homozygous tall plant (TT) is crossed with a heterozygous tall plant (Tt) T=tall, t=short 1. Key: TT X Tt T T 2. Cross: T TT TT 0% Probability of short pea plants: 100% Probability of tall pea plants: t Tt Tt
C: If round peas are dominant over wrinkled peas, make a Punnett square to determined the genotype and phenotype ratios of the offspring if a heterozygous plant is crossed with a homozygous recessive plant R=round, r=wrinkled 1. Key: Rr X rr R r 2. Cross: r Rr rr RR:Rr:rr 0:2:2 Genotypic ratio: 2:2 Phenotypic ratio: r Rr rr
D: Use a Punnett square to determine the genotype and phenotype ratios of the offspring from a cross between a homozygous dominant yellow pea plant and a homozygous recessive green pea plant Y=yellow, y=green 1. Key: YY X yy Y Y 2. Cross: y Yy Yy YY:Yy:yy 0:4:0 Genotypic ratio: 4:0 Phenotypic ratio: y Yy Yy
Dihybrid crosses The punnett squares we have been doing are known as monohybrid crosses, meaning that only one traits has been considered at a time. In a dihybrid cross, 2 different genes on 2 different are analyzed. chromosomes
Punnett Practice dihybrid crosses A-B 1. Key 2. Cross 3. Punnet square
A. Peas homozygous for round shape and heterozygous for color are crossed with heterozygous yellow peas, heterozygous for shape Key: R=round, r=wrinkled Y=yellow, y=green Cross: Genotypic ratio: Phenotypic ratio:
B. Key: G= gray body, g= black body; R=red eyes, r =black eyes Cross: GGRr X Ggrr What are the phenotypes of the parent fruit flies: Genotypic ratio: Phenotypic ratio:
Incomplete Dominance Neither allele has complete dominance over the other; heterozygous phenotype is a blend of the 2 homozygous phenotypes Ex: snapdragons R = red W = white RW = pink
Incomplete Dominance: cross a pink snapdragon with a white snapdragon 1. Key: 2. Cross: Phenotypic ratio: Genotypic ratio:
Incomplete Dominance: cross a red snapdragon with a white snapdragon 1. Key: 2. Cross: Phenotypic ratio: Genotypic ratio:
Codominance Codominance Both alleles share dominance and are always expressed if present. Ex: In chicken B = black feathers X W = white feathers BW = black AND white feathers
Codominance: cross a black chicken with a black and white chicken 1. Key: 2. Cross: Phenotypic ratio: Genotypic ratio:
Codominance: cross a white chicken with a black and white chicken 1. Key: 2. Cross: Phenotypic ratio: Genotypic ratio:
Polygenic Traits Many genes ; Many traits are controlled by more than one gene; have a variety of choices for expression. Ex: _hair color, eye color, skin tone
Multiple Alleles Many genes that have more than 2 alleles, although an individual only has 2 alleles for the gene. Ex: blood group. There are 3 possible alleles for this gene.
Suggested study questions p. 283 (1-5, 11, 12, 17-20)