Heredity is the passing of characteristics from parents to offspring Trait – a characteristic that is inherited Example: –Hair color is a characteristic –Blond, brown are traits Genetics – the study of heredity

Alleles – different forms of a gene or trait, there is one copy (allele) from mom and one copy (allele) from dad Ex. Hair color is a trait, and brown or blonde are different alleles What’s the difference between alleles and traits?

Alleles vs Traits A trait is a characteristic (can usually be seen with our eyes) –Example: black fur An allele is an alternative form of a gene. (alleles code for traits). –Example: BB is the allele for black fur

Alleles vs Genes allele – one version of a gene that governs a characteristic (such as fur color) –Example: BB gene – segment of DNA containing instructions for a trait

Gregor Mendel – Austrian monk who was the first person to study the passing on of characteristics from parents to offspring. He did this by crossing male and female pea plants and observing the results. P 1 generation – original parents F 1 generation – first filial (offspring) F 2 generation – second filial (offspring)  grandchildren

Garden pea plants reproduce sexually, which means that they produce male and female sex cells (gametes) Female part Transfer pollen Pollen grains Male parts Cross-pollination

During Mendel’s studies, he discovered that some traits disappeared and later reappeared How can tall plants produce short plants? Dominant trait – observed trait that “masks” or covers the recessive trait. Expressed as a capital letter. (example: D) Recessive trait – trait that disappeared, which is “masked” or covered by the dominant trait. Expressed as a lower case letter. (example: d) Ex: for height – T=tall, t = short

Each characteristic was true breeding, meaning that it always produced offspring of that trait when it self-pollinated. Each characteristic had 2 distinct traits:

Mendel selected a six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall. The first generation He cross-pollinated this tall pea plant with pollen from a short pea plant. All of the offspring grew to be as tall as the taller parent. TT x tt What would this cross look like?

Mendel allowed the tall plants in this F1 to self- pollinate. The second generation After the seeds formed, he planted them and counted more than 1000 plants in this F2. ¾ of the plants were as tall as the tall plants in the P and F1 generations.

Only 1 trait is seen in the F1, but then 2 traits are seen in the F2? Mendel’s answer: dominant and recessive “factors” (alleles) In every case, he found that one trait of a pair seemed to disappear in the F 1 generation, only to reappear unchanged in ¼ of the F 2 plants.

Two organisms can look alike but have different allele combinations. Phenotypes and Genotypes Law of segregation Tt  Tt cross F1F1 Tall plant TTT T t Tt tT t t t 3 Tall Short 1 Tall F2F2

Phenotype – physical appearance Genotype – genetic make-up genotype determines phenotype Homozygous – same alleles for a trait, ex: SS Heterozygous – different alleles for a trait, ex. Ss More Terms…

If a pea plant is TT  What is the phenotype? Is it heterozygous or homozygous? Homozygous dominant or recessive?

If a pea plant is Tt  What is the phenotype? Is it heterozygous or homozygous? Homozygous dominant or recessive?

If a pea plant is tt  What is the phenotype? Is it heterozygous or homozygous? Homozygous dominant or recessive?

Law of Segregation –alleles of the same trait separate into different gametes, so parents only pass on one allele for each trait to each offspring Law of Independent Assortment – genes for different traits separate independently of one another, so different traits are passed on independently from parents to offspring

Mendel and today Mendel’s work has held true to this day –Genes on chromosomes control traits –Allele - the coding for particular traits –We now use letters to show these different alleles One change = independent assortment only applies to genes that are far apart on a chromosome or on different chromosomes

probability Use to make predictions More trials = better Each trial is independent of the previous 50%, ½, 1:2

Punnett Square – used to predict the possible genotype of the offspring Monohybrid cross – involving only one characteristic Dihybrid cross – involving two different characteristics

The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side.

Let’s Practice… TT x tt Tt x tt Tt x TT tt x tt Tt x Tt Ratio:

Test-cross Used to determine an unknown genotype Cross unknown with homozygous recessive B – black fur b – white fur I have a black rabbit but I don’t know whether it is BB or Bb. How can I check?

Incomplete dominance When both alleles influence phenotype; neither is dominant. Offspring have a mixture of parental phenotypes Ex: pink flowers from red and white parents

codominance Both alleles are expressed; neither is dominant. But alleles don’t blend Ex: blood type

Relating back to meiosis…

The passing on of characteristics from parents to offspring is __________. D. allelic frequency C. pollination B. heredity A. genetics

Gametes are __________. D. fertilized cells that develop into adult organisms C. both male and female sex cells B. female sex cells A. male sex cells

How did Mendel explain the results of his cross between tall and short plants, depicted in the diagram? TT T T tt t t All tall plants F1F1 Tall plant Short plant

Which of the following genotypes represents a plant that is homozygous for height? D. tt C. tT B. Hh A. Tt