Genetics Using Punnett Squares

1. Genetics Using Punnett Squares

2. Early Genetics • The study of genetics began with observations made by Gregor Mendel. • After noticing that the flowers his pea plants were either violet or white, Mendel began to study the segregation of heritable traits. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants. Remember that Mendel worked almost 150 years ago when nobody knew about genes or even the structures (chromosomes) that carry genes.

3. Mendel’ Pea Plants Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible traits such as: Pod color Seed Color Plant Height Green Yellow Green Yellow Seed Shape Short Pod Shape Tall Wrinkled Round Smooth Pinched

4. Mendel’s Experiments Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent plant. He called these plants “purebred”plants. For instance, purebred short plants always produced short offspring and purebred tall plants always produced tall offspring. X Short Offspring Purebred Short Parents X Purebred Tall Parents Tall Offspring

5. Mendel’s First Experiment Mendel crossed purebred plants with opposite forms of a trait. He called these plants the parental generation , or P generation. For instance, purebred tall plants were crossed with purebred short plants. X Parent ShortP generation Parent TallP generation Offspring TallF1 generation Mendel observed that all of the offspring grew to be tall plants. None resembled the short short parent. He called this generation of offspring the first filial , or F1 generation, (The word filial means “son” in Latin.)

6. Mendel’s Second Experiment Mendel then crossed two of the offspring tall plants produced from his first experiment. Parent Plants Offspring X TallF1 generation 3⁄4 Tall & 1⁄4 ShortF2 generation Mendel called this second generation of plants the second filial, F2, generation. To his surprise, Mendel observed that this generation had a mix of tall and short plants. This occurred even though none of the F1 parents were short.

7. Mendel’s Law of Segregation Mendel’s first law, the Law of Segregation, has three parts. From his experiments, Mendel concluded that: 1. Plant traits are handed down through “hereditary factors” in the sperm and egg. 2. Because offspring obtain hereditary factors from both parents, each plant must contain two factors for every trait. 3. The factors in a pair segregate (separate) during the formation of sex cells, and each sperm or egg receives only one member of the pair.

8. Dominant and Recessive Genes Mendel went on to reason that one factor (gene) in a pair may mask, or hide, the other factor. For instance, in his first experiment, when he crossed a purebred tall plant with a purebred short plant, all offspring were tall. Although the F1 offspring all had both tall and short factors, they only displayed the tall factor. He concluded that the tallness factor masked the shortness factor. Today, scientists refer to the “factors” that control traits as genes. The different forms of a gene are called alleles. Alleles that mask or hide other alleles, such as the “tall” allele, are said to be dominant. A recessive allele, such as the short allele, is masked, or covered up, whenever the dominant allele is present.

9. Homozygous Genes What Mendel refered to as a “purebred” plant we now know this to mean that the plant has two identical genes for a particular trait. For instance, a purebred tall plant has two tall genes and a purebred short plant has two short genes. The modern scientific term for “purebred” is homozygous. short-short short-short short-short X Short Offspring Short Parents According to Mendel’s Law of Segregation, each parent donates one height gene to the offspring. Since each parent had only short genes to donate, all offspring will also have two short genes (homozygous) and will therefore be short.

10. Hybrid Alleles In Mendel’s first experiment, F1 offspring plants received one tall gene and one short gene from the parent plants. Therefore, all offspring contained both alleles, a short allele and a tall allele. When both alleles for a trait are present, the plant is said to be a hybrid for that trait. Today, we call hybrid alleles heterozygous. tall-tall short-tall short-tall short-short X Parent ShortP generation Parent TallP generation Offspring TallF1 generation Although the offspring have both a tall and a short allele, only the tall allele is expressed and is therefore dominant over short.

11. Dominant Alleles Mendel observed a variety of dominant alleles in pea plants other than the tall allele. For instance, hybrid plants for seed color always have yellow seeds. Green & Yellow Allele Yellow Seed However, a plant that is a hybrid for pod color always displays the green allele. Green Pod Green & Yellow Allele In addition, round seeds are dominant over wrinkled seeds, and smooth pods are dominant over wrinkled pods.

12. Law of Independent Assortment Mendel’s second law, the Law of Independent Assortment, states that each pair of genes separate independently of each other in the production of sex cells. For instance, consider an example of the following gene pairs: According to Mendels’ Law of Independent Assortment, the gene pairs will separate during the formation of egg or sperm cells. The plant will donate one allele from each pair. The plant will donate either a yellow or green seed allele, either a yellow or green pod allele, and a wrinkled or round seed allele. It will always donate a wrinkled pod shape. The donation of one allele from each pair is independent of any other pair. For example, if the plant donates the yellow seed allele it does not mean that it will also donate the yellow pod allele.

13. Lets consider a single gene… • A gene carries information that determines your traits. Traits are characteristics you inherit from your parents. • Genes are located in chromosomes. • Chromosomes come in pairs and there are thousands, of genes in one chromosome.

14. Continued… • In humans, a cell’s nucleus contains 46 individual chromosomes or 23 pairs of chromosomes. • Half of the chromosomes come from one parent and half come from the other parent. This is a human karyotype representing the 23 pairs of chromosomes in a male Here is the detailed structure of a chromosome

15. Definitions • Allele- discrete version of the same gene • Genotype- the genes of an organism for one specific trait • Phenotype- the physical appearance of a trait in an organism

16. Definitions • Dominant trait refers to a genetic feature that “hides” the recessive trait in the phenotype of an individual. • The term "recessive” describes a trait that is covered over (or dominated) by another form of that trait and seems to disappear. • Homozygous= two alleles that are the same for a trait (Pure) • Heterozygous= two different alleles for a trait (Hybrid)

17. Practice • We use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele). • Example below: P=dominant purple and p= recessive white The phenotype for this flower is violet while its genotype (if homozygous) is PP. The phenotype for this flower is white while its genotype is pp (to be white the flower must have two of the recessive copies of the allele).

18. Punnett Squares • The Punnett square is the standard way of working out what the possible offspring of two parents will be. • It is a helpful tool to show allelic combinations and predict offspring ratios.

19. Before we go further lets review how to set up a Punnett Square… We begin by constructing a grid of two perpendicular lines.

20. Next, put the genotype of one parent across the top and the other along the left side. For this example lets consider a genotype of BB crossed with bb. B B • Notice only one letter goes above each box • It does not matter which parent’s genotype goes on either side. b b

21. Next, fill in the boxes by copying the column and row head-letters down and across into the empty spaces. B B b b b B B b b B b B

22. Punnett Squares • Now that we have learned the basics of genetics lets walk through some examples using Punnett Squares.

23. W w Usually write the capital letter first W W W w W Lets say: W- dominant white w- recessive violet w W w w w Parents in this cross are heterozygous(Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have violet flowers? ANSWER: 25% (homozygous recessive)

24. Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond. R r r r 50% What percentage of the offspring will have red hair?

25. Let’s try some more… In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a short pea plant. T t What are the percentage of phenotypes? t t 50% tall 50% short

26. Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes. R R What is the possibility of a red eye off springs? r r 0% 

27. References • http://www.athro.com/evo/gen/punnett.html • http://www.kidshealth.org/kid/talk/qa/what_is_gene.html • http://brookings.k12.sd.us/biology/ch%2011%20genetics/punnettpractice.ppt#1 • http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sciber/punnett.htm • http://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg