Heredity and Genetics

1. Heredity and Genetics Heredity

2. Heredity • Definition = is the passing of traits (characteristics) from the parent to the offspring • Genetics is the study of heredity • Chromosomes and Genes • Made of DNA (the genetic material) • DNA carries genes that code for proteins • Proteins create traits

3. Heredity • There are 46 individual chromosomes in humans, 23 pairs • Homologous chromosomes carry genes for the same traits • 2 genes code for each trait, one on each homologous chromosome of the pair. • When gametes are formed, one copy of each homolog is placed in each sex cell= Law of Segregation

4. Heredity • During Meiosis One chromosome from the female and one chromosome from the male make a homologous pair sperm egg 23 23 23 23 23 23 23 23

5. Heredity • Of a pair of genes that codes for a trait, you get 1 gene from mom and 1 gene from dad. • If the 2 genes in the pair code for the same condition, then the individual is said to be homozygous for that trait. • Trait = eye brows, Two distinct eye brows from mom and Two distinct eye brows from dad • Use letter to represent genes T and T; written TT (Homozygous)

6. Heredity • If the 2 genes in the pair codes for different conditions, the individual is said to be heterozygous for the trait. • Trait = eye brows ;Two distinct eye brows from mom and (t) unibrow gene from dad; Tt • Use capital letters for the one gene and the lower case letter for the other, if different. Always use the same letter, just different case

7. Heredity • Genotype vs. Phenotype • Genotype = the types of genes present in the DNA. The genetic make up of the organism. Use letters to show the genotype. • Phenotype = The expressed traits of the organism. An organism’s outward appearance. Describe what they look like with words. • Genotype = TT or Tt or tt • Phenotype = Two eye brows or unibrow

8. Heredity • Sex Determination • 23 pairs of chromosomes in a normal human body cell • 23rd pair determines the sex • Gametes carry only 1 sex chromosome (1 of the original homologous pair) • Female gametes carry only X chromosomes, the female sex chromosome

9. XY male X Y + XX female X X + Heredity • Male gametes carry either an X or Y sex chromosome • During fertilization, when egg and sperm unite: The father is responsible for determining the sex of his offspring.

10. Heredity • The Law of Dominance and Recessives • Discovered by Gregor Mendel • Father of Genetics • Studied pea plants to make his observations • One gene of a gene pair can be dominantover the other (which is recessive). • The dominant gene will cover the presence of the recessive • Dominant gene is written as a capital letter. Use the 1st letter of the word for the trait. • Recessive gene is written as a lower case. Use the same letter as was used of the dominant gene.

11. Heredity • Example: Trait = Height • Tall is dominant; short is recessive • T = tall; t = short • GenotypePhenotype TT (homozygous) Tall Tt (heterozygous) Tall tt (homozygous) Short

12. Heredity • Example: Trait = Hair Color (Guinea Pigs) • Black is dominant over white • B = Black b = white • Genotype Phenotype BB (homozygous) Black Bb (heterozygous) Black bb White

13. Tt T t Genetics • Genes for a trait occur in pairs • During meiosis, one gene from a pair is placed in each gamete • Example: Germ cell (sex cell) with genes for height • A germ cell (gamete) can have either T or t

14. Genetics Each of the father’s genes go here C. Punnett Squares A chart used to show all possible gene combinations in a cross. Each of the mother’s genes for a trait go here Offspring: Possible combinations that could result

15. Suggestions • Begin by writing out the male and female parents’ genotypes. (If not already given) • Place male gametes on top of Punnett Square; outside the boxes • Place the female gametes along the left side of the Punnett Square; outside the boxes • Combine male and female gametes inside the boxes to show possible zygote combinations.

16. Summarize the 4 possible zygotes by listing the probabilities of genotypes and phenotypes. • Probabilities = fractions, percent, ratios • ¼ = 25%, ½ = 50%, ¾ = 75%

17. Genetics • Sample: heterozygous tall x homozygous tall Tt x TT Probability = the chance that a combination will occur Genotype TT = 50% Tt = 50% Phenotype Tall = 100% Short = 0% T T T t

18. Genetics • Practice Problems • Trait for seed color. Yellow seeds are dominant; green seeds are recessive. Cross a homozygous green seeded female plant with a homozygous yellow seeded male plant. yy x YY Probability Genotype Yy = 100% Phenotype Yellow = 100% Y Y y y

19. Genetics yy x Yy • Same trait Homozygous green female x heterozygous yellow male Y y Probability Genotype Yy = 50% (1) yy = 50% (1) Phenotype Yellow = 50% (1) Green = 50% (1) y y

20. Genetics Yy x Yy • Same trait Heterozygous yellow for both parents Y y Probability Genotype YY = 25% (1) Yy = 50% (2) yy = 25% (1) Phenotype 75% yellow (3) 25% green (1) Y y

21. Genetics Parents = XX x XY • What is the probability that an offspring will be male? Female? X Y Probability Genotype XX = 50% XY = 50% Phenotype Male = 50% Female = 50% X X

22. Genetics • Test Cross • If a dominant trait is present, the only way to know if the individual is homozygous or heterozygous without expensive testing, is to perform a test cross. • Test cross involves crossing a recessive individual with a dominant individual with unknown genotype. • If any of the offspring have the recessive trait, it is due to the dominant parent must be heterozygous. • Works best in organisms that have multiple offspring at the same time.

23. Mechanisms of Heredity Chromosome Theory • Says that the chromosome must be carriers of traits and that each chromosome could carry the genes for many traits. • It also proposed that alternate forms or alleles of a gene for a particular trait are located on homologous chromosomes.

24. Mechanisms of Heredity In 1905, it was discovered that in females, the sex chromosomes matched, but in males, they do not match. Today we assign XX to represent a female and XY to represent a male. In humans, the sex chromosomes make up one pair out of 23 pairs. The other 22 pairs of chromosomes are called autosomes or body chromosomes. These chromosomes do not play a part in sex determination in offspring.

25. Law of Independent Assortment • States that alleles of different genes separate independently of one another during gamete formation. • Example: gene for height is independent from gene for hair color

26. Sex Linked Genes • Some traits are carried on the sex chromosomes. Traits whose genes are located on sex chromosomes are called sex linked traits. • Most are carried on the X chromosome.

27. Video • Crash Course in Heredity

28. Incomplete Dominance • In incomplete dominance, you can tell the genotype by looking at the phenotype. • It is defined as the blending of differing traits. • No dominant or recessive trait. • Example: snapdragon colors • Red (RR) x Red (RR) = Red (RR) • White (WW) x White (WW) = White (WW) • Red (RR) x White (WW) = PINK (RW) • Neither red nor white are completely dominant. Because of this, when they are crossed, they appear pink, or an in between color.