Mendel and Meiosis Chapter 10 p

Chapter Outline  Mendel’s Laws of Heredity  Meiosis

Mendel’s Laws of Heredity Section 1 p

Genetics  Genetics is the branch of biology that studies heredity.  Heredity is the passing of traits from from parents to offspring.  one generation to the next  Traits are characteristics that are inherited.  Gregor Mendel is the father of genetics.  He studied pea plants.

Why peas?  Mendel chose peas because they reproduce sexually, and both the male and female gametes are in the same flower. Therefore, plants could be self-pollinated or cross-pollinated.  Gametes – sex cells (sperm & egg = pollen & ovule).  Gametes have only 1 copy of each chromosome,  But body cells have 2 copies of every chromosome (1 from mom, 1 from dad).

Pea Reproduction  Pollination – The transfer of the male pollen grains to the female ovule.  Fertilization – The fusing together of male and female gametes.  Self pollination – male and female gametes come from the same plant.  Cross pollination – male and female gametes come from different plants.

Mendel’s Monohybrid Cross  Monohybrid cross – Reproductive cross in which the parents differ by only one trait  Mendel cross-pollinated a true breeding tall and true breeding short pea plant to produce a new hybrid plant.  True breeding – when crossed with itself, produces only offspring like itself.  Hybrid – offspring of parents that have different forms of a certain trait.  All the F1 offspring of the monohybrid cross were tall.  F1 generation = 1 st offspring  P1 generation = parents  Then he self-pollinated the F1 generation  ¾ of the F2 offspring were tall, and ¼ were short.

Rule of Unit Factors  Mendel’s conclusion from his monohybrid cross:  Rule of unit factors: each organism has two factors for each trait.  He knew this because the short factor showed up in the F2 generation.  We now know that these factors are genes that are located on chromosomes.  One gene (or factor) comes from the female parent and one comes from the male parent.  Different forms of genes (factors) for the same trait are called alleles.  Each allele exists on a chromosome from mom and dad.  TT, tt, Tt

Phenotype and Genotype  Phenotype – the way a trait is expressed, its looks or behavior  Genotype – allele / genetic makeup of trait  Dominant Allele – upper case, written first  Recessive Allele – lower case, written last  Homozygous – both alleles are the same  TT (homozygous dominant)  tt (homozygous recessive)  Heterozygous – alleles are different  Tt

Rule of Dominance  Dominant traits  The dominant allele is the one that is always seen in the phenotype.  Tall plant allele is dominant over short plant allele.  Organisms can look the same but have different alleles (genes) due to dominance.  TT, Tt  Recessive traits  The recessive allele is only seen in the phenotype when no dominant allele is present.  tt

Law of Segregation  Every individual plant has two alleles of each gene. (Tt)  Gametes (sex cells) receive only one of these alleles, so two types of gametes can be produced. (Gamete 1 = T, Gamete 2 = t)  Then, fertilization combines one allele from each parent randomly.  In other words, the alleles of the same trait separate (or segregate) from one another and enter the gametes.  during fertilization, the gametes randomly pair to form 4 combinations of alleles

Mendel’s Dihybrid Crosses  Mendel also did crosses involving TWO different traits, these are called dihybrid crosses.  Pea Shape – Round or wrinkled  RR, Rr, rr  And Pea Color – Yellow or green  YY, Yy, yy

Dihybrid Cross  Parent Generation (P1)  All true breeding pea plants  RRYY x rryy  Second Generation (F1)  All Round and Yellow  Third Generation (F2)  9 Round Yellow, 3 Round Green, 3 Wrinkled Yellow, 1 Wrinkled Green  9:3:3:1 phenotypic ratio  This ratio always occurs in the F2 generation!

Law of Independent Assortment  Explains the 9:3:3:1 ratio in the F2 generation observed from Mendel’s dihybrid crosses.  Genes for different traits are inherited independently of each other. They don’t stick together.  RrYy produces gametes of any of the following type,  Ry  RY  rY  ry

Punnett Squares  Shorthand method of finding possible genotypes developed in 1905 by Reginald Punnett.  Used as an easy way to predict the phenotypic ratios of possible offspring.

Classwork – Due Today!  Section Assessment (1-5), pg. 262  Do these monohybrid cross Punnett squares:  1.) Homozygous tall x homozygous short  2.) Homozygous tall x heterozygous tall  3.) Heterozygous tall x homozygous short  Tell whether each of these represents genotype or phenotype. If it represents genotype, tell whether it is heterozygous, homozygous dominant, or homozygous recessive.  a.) LL  b.) blond hair  c.) dimpled chin  d.) blue eyes  e.) Dd  f.) ss

Meiosis Section 2

Chromosome Numbers  Diploid  A cell with two of each kind of chromosome  One set of chromosomes from each parent  2 sets of instructions (Tt)  2n  Haploid  Gamete cells contain ONE of each kind of chromosome  1 set of instructions (T) or (t)  1n

Homologous Chromosomes  Two chromosomes of each pair (one from each parent)  Genes arranged in same order on each chromosome  Genes come in different versions called alleles!

Meiosis produces haploid gametes  Mitosis = reproduction of body cells  Meiosis = production of sex cells (gametes)  A cell undergoes a normal cell cycle before a meiotic division - DNA is duplicated  Meiosis involves two separate divisions, producing four haploid cells  Each gamete produced is genetically unique

Crossing Over  Occurs during Prophase I  Two Chromatids, one from each pair, wrap around each other  Chromatids often break as they twist and the broken ends may switch places

Meiosis provides variation  7 pairs of chromosomes (Pea plant)  128 Possible sperm, 128 possible eggs  16,384 possible offspring  23 pairs of chromosomes (Human)  8 million possible sperm, 8 million eggs  70 trillion different zygotes possible

Nondisjunction  Failure of homologous chromosomes to separate properly  Trisomy – extra chromosome  Monosomy – missing chromosome  Sometimes entire sets don’t separate  Triploid,Tetraploid, Hexaploid  Called polyploids  Frequently occurs in plants

Gene Linkage and Maps  Sometimes genes seem to be inherited together  These genes lie very close on a chromosome to each other – Linked genes  Crossing over can separate these linked genes – frequency of new gene combinations help map genes