Genetics 2 Mendel Inheritance What is Inheritance Most

Genetics #2: Mendel & Inheritance?

What is Inheritance? • Most people think about inheriting money from a relative who has died. • But, we’re going to learn about another form of inheritance…. GENES!

What has genes? • Genetics is the scientific study of heredity, or where we get our genes from and how. • Every living thing has genes, they are responsible for certain sets of characteristics which get passed from the parent(s) to the offspring.

Who discovered Genetics? • Gregor Mendel discovered genetics. – Austrian monk – Went to school for science and math – Taught high school – Was in charge of the monastery garden • He started playing with garden pea plants and discovered different modes of inheritance

What he knew • Part of each pea plant produces pollen which contains the male reproductive cells, sperm. • The female portion of the plant produces egg cells. • During sexual reproduction the male and female reproductive cells join, this is called fertilization.

• The result of fertilization is a tiny embryo within a seed. • Pea flowers are normally self-pollinating which means that the pollen from the flower normally fertilizes the egg in the same flower • The seeds produced from a self-pollinating plant bear all the characteristics of their parent plant, therefore they have a single parent.

Here comes Mendel! • When Mendel took over the monastery garden there were several types of pea plants. • They produced different characteristics like tallness, shortness, green seeds and yellow seeds. • The plants were true-breeding, which means if they were allowed to self-pollinate they would produce offspring identical to themselves

• Mendel wanted to produce seeds by joining male and female sperm and eggs from different pea plants. • Mendel started crosspollinating. He started by cutting off the pollen-bearing male parts in a pea flower, then he dusted pollen from another plant onto the flower. This produces seeds with 2 different parents.

• A trait is a characteristic like seed color or plant height that varies from one individual to another. • Mendel studied the effects of crossing 7 plants with different traits. • The original plants are called the P (parental) generation and the offspring are called F 1 (first filal or “son”) • The offspring of crosses with different parents are called hybrids.

Results from the cross • Were the F 1 generation plants a blended mix of both parents’ characteristics? NO! • It turns out the F 1 generation plants had either the characteristic of one parent OR the other, but not both!

Mendel’s 2 Conclusions 1. Biological inheritance is determined by factors that are passed from one generation to the next. • We call the chemical factors that determine traits, genes. • Each trait Mendel studied was controlled by one gene that occurred in 2 contrasting forms • The different forms of a gene are called alleles.

Mendel’s 2 Conclusions 2. The principle of dominance states that some alleles are dominant while others are recessive. • An organism with the dominant form of a trait will always exhibit that form of the trait. An organism with the recessive form of the trait will exhibit it only if the dominant trait is NOT present.

Segregation • Mendel wondered, “have the recessive alleles disappeared or are they still present in the F 1 generation? ” • He allowed F 1 generation to self-pollinate and produce an F 2 generation shown below:

What he discovered… • When the F 1 generation was allowed to self-pollinate the recessive alleles reappeared in the F 2 generation! • So why did the recessive alleles seem to disappear in the F 1 generation? Let’s take a closer look.

Turns out… • The alleles segregate from each other during gamete formation when crossingover occurs so that each gamete carries only a single copy of each gene. • Each F 1 plant produces 2 types of gametes– those with the allele for tallness and those with the allele for shortness.

• Crossing over - during meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. • These points of contact can allow the exchange of genetic information between chromosomes.

Crossing over occurs during prophase I and means that each daughter cell formed in meiosis has chromatids with a different combination of alleles.

Probability and Punnett Squares • Mendel discovered that the principles of probability could be used to explain results of genetic crosses – Example: He realized every time he crossed 2 plants hybrid for stem height (Tt), about ¾ of offspring were tall and ¼ were short! • The gene combinations that might result from a genetic cross can be determined by drawing a punnett square. Punnett squares can be used to predict and compare the genetic variations that will result from cross.

Punnett Square • The gametes produced by each F 1 parent are along the top and left side of the square. The possible gene combinations for the F 2 generation appear in the 4 boxes that make up the square. – The letters in the boxes represent alleles, T is the dominant allele for tallness and t is the recessive allele for shortness

remember • Organisms that have 2 identical alleles for a particular trait are called homozygous (TT or tt). – True-breeding • Organisms that have 2 different alleles for a particular trait are called heterozygous (Tt). – Hybrid

Break it down… • All of the tall plants have the same phenotype, or physical characteristics. • But, they do not have the same genotype, or genetic makeup. • This makes the ratio of tall to short plants 3: 1

Independent Assortment • Mendel performed a two-factor cross • He first crossed true-breeding plants that produced only round yellow (RRYY) peas with plants that produced wrinkled green peas (rryy) • All F 1 offspring are round yellow peas, showing those are the dominant alleles (Rr. Yy)

What’s Next? • Mendel knew all F 1 plants were heterozygous for both seed shape and color (Rr. Yy) • F 1 s were crossed with each other to produce F 2 generation • Independent allele segregation resulted

F 2 generation results • The F 2 plants produced 566 seeds. – 315 seeds were round and yellow – 32 seeds were wrinkled and green – 209 seeds had combinations of the phenotypes • Independent assortment occurred. The alleles for seed color and shape segregated independently. • The results were close to a 9: 3: 3: 1 ratio

Independent Assortment • The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. • Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms.

Summary of Mendel’s Principles • Inheritance of biological characteristics is determined by genes • In cases where 2 or more alleles of a gene for a single trait exist, some are dominant and some are recessive • Most parents have 2 copies of a single gene which segregate when gametes are formed • Alleles for different genes usually segregate independently of one another

It’s not black and white • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes – Incomplete dominance- cases in which one allele is not completely dominant over another (blended phenotype) – Codominance- both alleles contribute to the phenotype (spotty or checkered phenotype)

Its not black & white – Multiple Alleles- more than 2 alleles which when they segregate can result in a wider variation of phenotypes (bunny fur color, blood type) – Polygenic traits- traits controlled by 2 or more genes (human skin color)

• The characteristics of any organism are not determined solely by the genes it inherits. • The characteristics are determined by interaction between genes and the environment – Genes may affect a sunflower’s color and height, but the same factors are also influenced by climate, soil conditions and the availability of water

YOUR TURN! • Fill out the punnett square at the bottom of your notes. • We will review the answers together! #1 – Flower petal colors

http: //www. youtube. com/watch? v=d 4 iz. VAkh. MPQ&feature=related Your Turn Answers

#2 Can you predict chin shape? • 2 parents with cleft chins, both heterozygous for cleft chin (Tt) have 3 children with cleft chins. What is the probability that a fourth child will have a cleft chin? • Fill in a punnett square to determine the possible phenotype for the fourth child