1. Genetics Using Punnett Squares

2. Introduction to Punnett Squares!

3. Gregor Mendel 1800s – Monk, scientist, teacher Explored the laws of genetics through studying pea plants Looked at: – Seed color (yellow or green) – Stem length (long or short) – Flower color (white or purple

4. Some Rules: When Mendel proposed that each trait is determined by a pair of genes, it presented a problem. If parents pass on both copies of a gene pair, then offspring would end up with four genes for each trait.

5. The Principle of Segregation (First Law) Mendel deduced that sex cells — sperm and eggs — contain only half of the genes from each parent. The two alleles of a gene pair separate from each other in the formation of gametes (sperm and egg). Half the gametes carry one allele, and the other half carry the other allele.

6. The Principle of Independent Assortment (Second Law): Genes for different traits distribute independently of one another when gametes are formed.

8. Law of Independent Probability The chances of inheriting each trait is random every time a gamete (an egg or sperm cell) is formed

9. Mendel also found that different gene combinations from the parents resulted in specific ratios of dominant-to- recessive traits. For example, a cross between two purple hybrids produces three times as many purple flowers as white flowers. This is Mendel's famous 3 to 1 ratio.

10. Punnett Screencast based on animation fount at http://www.dnaftb.org/5/animation.htmlhttp://www.dnaftb.org/5/animation.html

11. Let’s not forget! The 3 to 1 ratio indicates probability (how likely it is to obtain a trait), NOT that for every four crosses there will be three yellow peas there will be one green pea. The result of each pairing is an independent event. Like the tossing of a coin...

12. 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 combinations of alleles and predict offspring ratios.

14. 2. 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 for the Dad, and a genotype of bb for the Mom. B = big ears b = small ears B B bbbb Notice only one letter goes above each box It does not matter which parent’s genotype goes on either side, as long as it is for the same parent.

15. 3. 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 Bb b b b b

16. Analysis: 1.What are the genotypes of the parents? – Mom: – Dad: 2.What are the phenotypes of the parents? – Mom: – Dad: 3.What are the genotypes of the offspring? 4.What are the phenotypes of the offspring? 5.What are the chances that a child will have big ears? Small ears? How do you know?

17. Punnett Squares ■ Now that we have learned the basics of genetics let’s walk through some examples to predict probability using Punnett Squares.

18. 1.In flowers, being white (W) is dominant over being purple (w). In a cross of two heterozygous white plants, what percentage of the offspring will have purple flowers?

19. 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? Usually write the capital letter first w w ww w ANSWER: One out of four (1/4) or 25% (homozygous recessive)

20. 2. Red hair (R) is dominant over blond hair (r). In a cross between a heterozygous redhead and a blonde, what percentage of the offspring will have red hair? R r rrrr Redhead Blonde

21. Let’s try some more… 3. 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 tttt What are the percentages of possible offspring phenotypes?

22. 4. 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 rrrr What is the possibility of a red eye offspring?

23. Your Turn! Complete # 1-3 of the classwork in your packet – page #4 This uses traits from the “Making Faces” lab from last class – you may refer back to it!

24. References ■ http://www.athro.com/evo/gen/punnett.html http://www.athro.com/evo/gen/punnett.html ■ http://www.kidshealth.org/kid/talk/qa/what_is_gene.html http://www.kidshealth.org/kid/talk/qa/what_is_gene.html ■ http://brookings.k12.sd.us/biology/ch%2011%20genetics/punnettpr actice.ppt#1 ■ http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sci ber/punnett.htm http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sci ber/punnett.htm ■ http://www.biotechnologyonline.gov.au/images/contentpages/karyot ype.jpg