Mendel’s Laws of Heredity Section 10.1 p. 253 - 262 Chapter 10 Mendel and Meiosis

Mendel chose which pea plants he would allow to fertilize or pollinate one another and then kept records of the offspring produced.

Before we can go any further… There are a bunch of vocab words we need to start using. Alleles Dominant Recessive Homozygous Heterozygous

Mendel’s ~ The Rule of Unit Factors Each organism has two factors that control each of its traits. These factors are now known as genes. Genes come in different forms. Alternative forms of a gene for each variation of a trait is an allele.

alleles We get two copies of each allele or directions for a trait. One copy comes from each parent. We may get a matching set or they may be different with the dominant one hiding or masking the recessive version.

dominant Observed trait of an organism that masks the recessive form of a trait. If present, this is the trait we see. “Dominant” is not always “good”.

recessive Trait of an organism that can be masked by the dominant form of a trait. You need two copies, one from each parent, for a recessive trait to be expressed. Recessive traits are not always “bad”.

Mendel’s ~ The Rule of Dominance Even though an organism carries two alleles for the same trait, only one trait is observed. The dominant trait will be expressed.

Homozygous Having two of the same alleles for a gene When there are two identical alleles for a trait.

Heterozygous Having two different alleles for the same gene. Back to the peas, each pea has two height genes.  T = tall, t = short TT and tt are homozygous.  Tt is heterozygous. 

Phenotype Outward appearance of an organism, regardless of its genes. An organism's traits (like green vs. yellow seeds, or pea plant height) What you look like!

Genotype Combination of genes in an organism. Can not tell from outward appearance Set of directions on the inside.

Genotype If it is a recessive trait, the genotype is obvious. Dominant traits are expressed whether the organism is heterozygous (2 different alleles) or homozygous (2 of same alleles) so you do not know for sure just from looking.

hybrid Offspring formed by parents having different forms of a trait. Mendel’s crossbreed strains were made breeding know true-breeding plants to different true-breeding plants. Working with his peas, Mendel used 355 cross-bred strains and 12,980 resultant hybrids.

True-breeding or homozygous True-breeding means that they only have one version of a trait to pass on or homozygous. Sometimes we refer to homozygous organisms as being a “pure bred”. If a pea plant came from parents that were tall, it is tall and all of its offspring are tall when crossed with other pea plants that are “true-breeding”, we can be fairly certain that it is homozygous.

Poodles

Wolves

What would their puppies look like? In 1976 a person named C. Schleifenbaum lead a study to find out.

poodle-wolf hybrids They bred wolves to poodles. Then looked at the fur marks and coat color in wolves and poodle-wolf hybrids. Here are some pictures of their results.  (Zeitschrift fuer Saeugetierkunde, 41(3):147-167. BA 62:60461    ZR 113(19):148).

1st generation ~ poodle-wolf hybrid

2nd generation ~ poodle-wolf hybrid Notice how the coats range from wolf-like to poodle-like

Monohybrid cross How can we explain the poodle-wolf hybrid results? Poodles are “true-breeding” for one version of traits. Wolves have a different version of these same traits.

Monohybrid cross In a monohybrid cross, the parents selected are true-breeding for the trait we will be studying. The parents are known as the P1 generation. Two P1 parents will be breed or crossed and the resulting offspring are the 1st or F1 generation.

1st generation First generation is the F1 generation “F” stands for filial ~ son or daughter In the F1 generation, all of the offspring should be similar, showing all the dominant traits that were passed on by the P1 generation.

1st generation All of the poodle-wolf hybrids looked the same.

Mendel’s ~ The Law of Segregation Each plant has two different alleles, it can produce two different types of gametes. During fertilization, male and female gametes randomly pair to produce four combinations of alleles.

The Law of Segregation

Monohybrid Crosses Homozygous P1 = All offspring will have the same genotype They will all have the same phenotypes & look like the dominant trait. Heterozygous P1 = The offspring will have a 1:2:1 ration in genotypes 3:1 ration in phenotypes

Homozygous crosses Mendel choose plants that were breeding true. His peas must have been homozygous… Only able to provide one type of allele or variety of that particular trait.

Homozygous crosses Mendel’s first pea plant crosses were between tall pea plants and short pea plants. One parent has two tall alleles and the other parent has two short alleles. They are both homozygous for their trait.

How we represent the parents & offspring in a cross A letter is selected to represent a trait. The dominant version is shown with the capital. The recessive version is shown with the lower case .

How we represent the parents in a cross With the height of pea plants a capital T is used to represent the tall version A lower case t is used to represent the short version

Genotypes in the cross The tall pea parents will be = TT, it can only give T alleles The short pea parents will be = tt, it can only give t alleles

Punnett Squares A punnett square is a way to predict the possible outcome of a breeding cross Each box represents a reproductive opportunity or offspring. Short parent’s alleles t Tall parent’s alleles T T

Punnett Squares The offspring gets one allele from each parent so they end up with two copies of alleles for each trait. Divide each parent’s two alleles so that each box has one letter next to or above it Short parent’s alleles t Tall parent’s alleles T T

Monohybrid cross - Homozygous If one parent is homozygous dominant for their trait and the other is homozygous recessive, the offspring will all have the same phenotype and genotype. Short parent’s alleles t Tall parent’s alleles T T

Monohybrid cross - Homozygous The offspring’s genotype (what the alleles say) will be = Tt The offspring’s phenotype (what we see) will be = The plants will be tall. The ________ is the dominant trait. Short parent’s alleles t Tall parent’s alleles T T

t t T t T t T T t T t T Monohybrid cross Both Homozygous/ 1 –Recessive 1 -Dominant t t Genotype Genotype T t T t T Phenotype = Phenotype = Dominant Dominant Genotype Genotype T t T t T Phenotype = Phenotype = Dominant Dominant

2nd generation Second generation is the F2 generation The second generation all has two different copies (heterozygous) for each trait. Their offspring will show more variation because the alleles will regroup in many ways.

Monohybrid cross - Heterozygous If two organisms are selected out of the F1 generation to be parents for the next generation, they will each be heterozygous. Sticking with our offspring of a tall pea plant & and a short pea plant, this group will have a genotype of Tt. They will have a phenotype of tall.

Monohybrid cross - Heterozygous Remember, the offspring get one allele from each parent so they end up with two copies of alleles for each trait. Both parents are heterozygous so they have a genotype of Tt. Heterozygous parent’s alleles T t Heterozygous parent’s alleles T

Monohybrid cross - Heterozygous How many of the offspring will be tall? How many of the offspring will be short? Heterozygous parent’s alleles T t Heterozygous parent’s alleles T

Monohybrid cross - Heterozygous What are the possible genotypes for the offspring? What is the ratio of tall (dominant) to short (recessive) plants in the F2 generation? Heterozygous parent’s alleles T t Heterozygous parent’s alleles T

T t T T T t T t T t t t Monohybrid cross Both Heterozygous Genotype Phenotype = Phenotype = Dominant Dominant Genotype Genotype t T t t t Phenotype = Phenotype = Dominant Recessive

2nd generation The second generation of poodle-wolf hybrids looks very different from each other!

Crossing Homozygous Pea Plants

Crossing Heterozygous Pea Plants