Introduction to Genetics Chapter 11 - Biology

Gregor Mendel 1822-1884 - The father of genetics Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments True-breeding peas – self-pollinate Wanted to stop self pollination he cut away the pollen-bearing male parts and dusted pollen from another plant onto the flower – cross-pollination

Mendel’s 7 Traits Trait – specific characteristic, such as seed color or plant height – makes one different from the other Mendel crossed plants with each of the 7 traits and studied their offspring (hybrids) P (parental) – original pair of plants F1 (first filial) – offspring Filius and filia are Latin for “son” and “daughter”

Mendel’s 7 Traits cont. He first concluded that biological inheritance is determined by factors that are passed from one generation to the next. Factors that determine traits are genes Different forms of a gene are called alleles His second conclusion: the principle of dominance (some alleles are dominant & others are recessive)

Mendel’s 7 Traits cont. Did the recessive alleles disappear, or were they still present in the F1 plants? 7 kinds of F1 to produce an F2 through self-pollination Recessive alleles disappeared in F1 and reappeared in F2 Alleles in F1 separated from each other during the formation of sex cells (aka gametes)

When similar crosses were repeated, similar results were obtained ie. Crossed 2 plants that were hybrid for stem height (Tt), about ¾ were tall and ¼ were short Probability – likelihood that a particular event will occur This can be used to predict the outcomes of genetic crosses

Punnett Square Chart that shows all the possible combinations of alleles that can result from a genetic cross used to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome T t TT Tt T Tt tt t

Coin Toss Lab Please take out your handout We will use the remainder of the period to work on and complete this lab

How are dimples inherited? Page 268 in Biology Text Book 1. Write the last 4 digits of your telephone number. These 4 random digits represent the alleles of a gene that determines whether a person will have dimples. Odd = allele for the dominant trait of dimples Even = allele for the recessive trait of no dimples 2. First 2 digits represent a certain father’s genotype. Symbols D and d to write his genotype.

3. Use the last 2 digits the same way to find the mother’s genotype. 4. Construct a Punnett square for the cross of these parents. Then, using the Punnett square, determine the probability that their child will have dimples. 5. Determine the class average of the percent of children with dimples.

Homozygous – organisms that have 2 identical alleles for a particular trait (ie. TT or tt) – true-breeding. Heterozygous – organisms that have 2 different alleles for the same trait (ie. Tt) – hybrid.

Phenotype – physical characteristics Things you can see Genotype – genetic makeup Things you cannot see – ie. genes (made of DNA) on chromosomes Mendel performed an experiment to see if seed shape affects seed color – crossed plants and recorded the two traits One did not effect the other (independent assortment) – genes for different traits can segregate independently during formation of gametes.

Summary of Mendel’s Principles Inheritance of biological characteristics is determined by genes. When two of more forms of the gene for a single trait exist, some forms may be dominant and others recessive. In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These are segregated from each other when gametes are formed. Alleles for different genes usually segregate independently of one another.

Dominant & Recessive Alleles Cases in which one allele is not completely dominant over another are called incomplete dominance (heterozygous phenotype is between the two homozygous phenotypes. Codominance – both alleles contribute to the phenotype. (ie. in chickens, the alleles for black feathers is codominant with the allele for white feathers.

Incomplete dominance - neither allele is dominant, red x white = pink Codominance - both are expressed in some way, red x white = white/red spots

Many genes have more than two alleles, known as multiple alleles. Does not mean that an individual can have more than two alleles Means that more than two possible alleles exist in a population – ie. rabbit coat color Traits controlled by two or more genes are said to be polygenic traits – “having many genes.” Often show a wide range of phenotypes ie. wide range of skin color in humans – more than 4 different genes control this trait

Chromosomal Inheritance Mendel’s principles of genetics requires; Each organism must inherit a single copy of every gene from each “parent” When an organism produces its own gametes, those 2 sets of genes must be separated so that each gamete contains just one set of genes ie. Body cell in an adult fruit fly has 8 chromosomes (4 from male parent, and 4 from female parent) – these two sets of chromosomes are homologous

A cell that contains both sets of homologous chromosomes is said to be diploid – 2 sets Represented by the symbol 2N ie. For the fruit fly the diploid number is 8 or 2N=8 Diploid cells contain two complete sets of chromosomes and two complete sets of genes Single set of chromosomes – single set of genes are haploid – 1 set or N=4 for a fruit fly

Phases of Meiosis Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Usually involves 2 distinct divisions known as meiosis I and meiosis II

Meiosis I Before meiosis I, each chromosome is replicated. Cells start to divide – 4 chromosomes line up in the middle of the cell and 2 chromatids that make up each chromosome separate Prophase – each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad 4 chromatids in a tetrad As tetrads are formed, they exchange portions of their chromatids in a process called crossing-over

Meiosis I cont… Crossing-over, results in the exchange of alleles between homologous chromosomes and produces new combos of alleles Homologous chromosomes separate, and two new cells are formed SEE PAGE 276 FIG. 11-15 (MEIOSIS)

Meiosis II 2 cells produced by meiosis I now enter a second meiotic division Each of the cell’s chromosomes has 2 chromatids Metaphase II - chromosomes line up in the center of each cell Anaphase II – paired chromatids separate

See Page 278 Fig. 11-17 Meiosis produces four genetically different haploid cells In males, meiosis results in four equal-sized gametes called sperm In females, only one large egg cell results from meiosis (the other 3 cells are polar bodies and are not involved in reproduction)

Mitosis vs. Meiosis Mitosis results in the production of two genetically identical diploid cells Allows an organism’s body to grow and replace cells In asexual reproduction, a new organism is produced by mitosis of the cell or cells of the parent organism Meiosis produces four genetically different haploid cells Begins with a diploid cell but produces four haploid (N) cells Is how sexually reproducing organisms produce gametes

Gene Linkage Thomas Hunt Morgan researched fruit flies which led to the principle of linkage Each fruit fly has four pairs of chromosomes – each chromosome is a group of genes It is the chromosomes that assort independently, not the genes If two genes are found on the same chromosome they are not linked forever - crossing-over during meiosis sometimes separates genes