Mendel and Meiosis Unit 4 Chapter 10

Gregor Mendel Austrian monk Studied patterns of heredity (passing on of characteristics from parent to offspring) Used the common garden pea in experiments

Why did Mendel use peas? Sexually reproducing: able to isolate both male and female gametes Easy to identify traits (characteristics that are inherited) Short life cycle: able to be grown quickly

Hybrid Any offspring of parents with different traits (ex: tall plant x short plant) Monohybrid cross: cross-pollination (breeding) between two parents with only one variation difference (ex: tall plant x short plant) Dihybrid cross: cross-pollination (breeding) between two parents with two variation differences (ex: tall, green plant x short, yellow plant)

Pea cross-pollination experiments PARENT GENERATION (P1) Tall true breed x short true breed FILIAL GENERATION (F1) All tall hybrids FILIAL GENERATION (F2) 75% tall hybrids, 25% short hybrids

Phenotypes from P1 to F2 Dihybrid Cross round yellow x wrinkled green All round yellow F1 F2 9 3 3 1 Round yellow Round green Wrinkled yellow Wrinkled green

What did Mendel observe? When a true-breeding tall plant is crossed with a true-breeding short plant in the P generation, the F1 height trait is always predictable. 100% are tall plants. P generation F1 F2

What happens when the F1 tall plants are crossed together? Mendel observed that the F2 generation, the offspring of F1 plants, are always in a fixed ratio of 3:1 tall:short. Why? P generation F1 F2

Pea traits that Mendel identified Through multiple crosses, Mendel determined that all these traits displayed a mathematical predictability for inheritance. Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall

Mendel’s conclusions There must be two variations for every trait, where each variation is called an allele. Each offspring inherits only one allele from each parent. The alleles are either dominant or recessive. To show the recessive trait, two recessive alleles must be inherited.

Dominant and recessive traits The traits that seem to mask other traits when present are called dominant traits. The traits that seem to be hidden in the presence of dominant traits are called recessive traits.

Dominant and recessive traits Seed shape Seed color Flower color Flower position Pod color Pod shape Plant height Dominant trait axial (side) round yellow purple green inflated tall Recessive trait terminal (tips) wrinkled green white yellow constricted short

Homozygous vs. Heterozygous Homozygous: inherits two similar alleles from the parents for a particular gene Ex: tall allele and tall allele, written as TT Ex: short allele and short allele written as tt Heterozygous: inherits two different alleles from the parents for a particular gene Ex: tall allele and short allele, written as Tt

Law of Segregation Mendel concluded only one allele is passed from parent to offspring for each trait. F1 plants must be heterozygous because the P generation only passed on one tall allele and one short allele. The F1 plant will then pass on to its offspring either a tall or a short allele, never both.

A a A a Using a Punnett square AA x aa  100% Aa Each of the four squares represents 25% chance of inheritance for one offspring. A a A a

Phenotype vs. Genotype Phenotype: physical appearance of the trait Ex: purple flowers Genotype: homozygous or heterozygous inheritance Ex: PP, Pp, pp

Law of independent assortment Because organisms are made up of more than one trait, Mendel concluded that the inheritance of one trait does not influence the inheritance of a second trait. Example: Height of the pea plant does not influence the color of the peas Height is independently assorted from color.

Using dihybrid crosses to show independent assortment A smooth, yellow pea (RrYy) can pass on these combinations of genes to its offspring: RY, Ry, rY, or ry.

Section 10.1 Summary – pages 253-262 Punnett Square of Dihybrid Cross Dihybrid crosses Gametes from RrYy parent RY Ry rY ry RRYY RRYy RrYY RrYy A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. RY RRYy RRYy RrYy Rryy Ry Gametes from RrYy parent RrYY RrYy rrYY rrYy rY RrYy Rryy rrYy rryy ry Section 10.1 Summary – pages 253-262

Dihybrid crosses Punnett Square of Dihybrid Cross Gametes from RrYy parent RY Ry rY ry RRYY RRYy RrYY RrYy RY F1 cross: RrYy ´ RrYy RRYy RRYy RrYy Rryy Ry round yellow Gametes from RrYy parent RrYY RrYy rrYY rrYy round green rY wrinkled yellow RrYy Rryy rrYy rryy ry wrinkled green

Modernizing Mendelian genetics DNA is the basis for inheritance. DNA are coiled into chromosomes. Parts of the DNA that code for a trait are called genes. Some genes have only two alleles and other have more. Gene for hairline Allele: A Genotype: Aa Gene for hairline Allele: a

How do these pictures compare?

Variations on Mendel Incomplete dominance: the heterozygous genotype shows a blend of the two parents and not the dominant allele

Variations on Mendel Codominance: the heterozygous genotype shows both inherited alleles Example of roan horse coat: AA (dark red) x aa (white)  Aa (dark red and white)

Variations on Mendel Multiple alleles: when there are more than two alleles that code for a trait Example: ABO blood type A type = AA or Ao B type = BB or Bo O type = oo AB type = AB

Blood typing

Variations on Mendel Polygenic trait: when more than one gene codes for a particular trait Example: fur color, human height, human skin color, eye color

Variations on Mendel Linked genes: Mendel concluded that traits are assorted independently, but some traits are linked. This means that two genes are almost always inherited together (ex: red hair, green eyes).

Cells and chromosomes A cell with two of each kind of chromosome is called a diploid cell and has diploid, or 2n, number of chromosomes. Organisms produce gametes that contain one of each kind of chromosome

Homologous chromosomes The two chromosomes of each pair in a diploid cell are called homologous chromosomes.

Homologous chromosomes On homologous chromosomes, the same types of genes are arranged in the same order. Because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other.

Making haploid cells Meiosis is the process of producing haploid gametes with a ½ the amount of DNA as the parent cell. A cell with one of each kind of chromosome is called a haploid cell and has a haploid, or n, number of chromosomes. Meiosis enables sexual reproduction to occur.

Mitosis and Development Sexual reproduction Haploid gametes (n=23) Sperm Cell Meiosis Meiosis Egg Cell Fertilization Diploid zygote (2n=46) Mitosis and Development Multicellular diploid adults (2n=46)

Interphase During interphase, the cell replicates its chromosomes. After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

Prophase I The chromosomes coil up and a spindle forms. Homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

Prophase I – crossing over Chromatids are wrapped so tightly the chromosomes can actually break and exchange genetic material in a process known as crossing over. Crossing over results in new combinations of alleles on a chromosome.

Metaphase I The centromere of each chromosome attaches to a spindle fiber. The spindle fibers pull the tetrads into the middle, or equator, of the spindle.

Anaphase I Homologous chromosomes separate and move to opposite ends of the cell. This critical step ensures that each new cell will receive only one chromosome from each homologous pair.

Telophase I The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells. Each cell has half the DNA as the original cell because it has only one chromosome from each homologous pair.

Prophase II A spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

Metaphase II. The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

Anaphase II The centromere of each chromosome splits. The sister chromatids to separate and move to opposite poles.

Telophase II Finally nuclei reform, the spindles breakdown, and the cytoplasm divides. Four haploid cells have been formed from one diploid cell

Why meiosis is important Forms gametes for sexual reproduction Crossing over during meiosis which rearranges allele combinations so that the offspring generations are genetically different than the parents.

Nondisjunction leading to trisomy This can lead to gamete formations having too many or too few chromosomes. Ex: A gamete with 2 copies of #21 chromosome fertilizes a gamete with 1 copy of #21. The result is an embryo with trisomy 21. This causes Down Syndrome in humans.

Trisomy leading to monosomy A gamete with one copy of the X chromosome fertilizes a gamete missing a copy of the X chromosome. The result is monosomy X, which in humans causes Turner Syndrome. Affects 1 in every 2,500 girls. Most girls with Turner Syndrome are infertile.

Nondisjunction leading to polyploidy When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid. The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes and is a tetraploid. This occurs often in flowering plants, leading to larger fruit production.