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Unit 6 Genetics: the science of heredity

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1 Unit 6 Genetics: the science of heredity
Heredity – the passing of traits from parents to offspring

2 Gregor Mendel – the Father of Genetics 1822-1884

3 Mendel’s Experiments Studied garden peas – 7 different traits with clearly different forms

4 Male and female parts in same flower Self pollination
Purple parents produce purple offspring White produce white

5 Cross Pollination

6 Parent First filial Crossed 2 F1 plants to get F2 Second Filial

7 Mendel’s Principle of Dominance
One form of a hereditary trait – the dominant trait – prevents the expression of the other – the recessive trait Eg. In peas, purple x white gives all purple offspring Purple is dominant White is recessive


9 Punnett Squares Show all possible resulting offspring from a cross between 2 parents AND the probability of each offspring Each parent contributes 2 genes for a trait (alleles) Represent with letters Capital = dominant Lower case = recessive Homozygous – alleles same eg. AA or aa Heterozygous – alleles different eg. Aa


11 Genotype vs. Phenotype Genotype = actual genetic make-up an individual has – represented by letters Phenotype = outward expression of the genotype – the protein that is produced. Eg. Let P = purple, and p = white Genotypes PP and Pp both have the same phenotype purple PP = homozygous dominant Pp = heterozygous pp = homozygous recessive

12 Genotypic ratio = probable ratio of genotypes in offspring of a cross
Eg. 1PP : 2Pp : 1 pp Phenotypic ratio = probable ratio of phenotypes resulting from the genotypic ratio Eg. 3 purple : 1 white

13 Observed ratio = what actually occurs
Expected ratio = the ratio expected based on probability (Punnett Square) Observed ratio = what actually occurs Why would these be different?

14 Phenotype genotype


16 Mendel’s Principle of Segregation
During gamete formation, the pair of genes responsible for each trait separates so that each gamete receives only one gene for each trait. This happens during meiosis I when homologous chromosomes separate (anaphase I)

17 Dihybrid Cross 2 traits with 2 forms


19 Genes on different chromosomes sort independently into gametes
Genes on different chromosomes sort independently into gametes. What if they are on the same chromosome?

20 Mendel’s Principle of Independent Assortment
Genes for different traits segregate independently during gamete formation when they are located on different chromosomes.. If they are on the same pair of homologous chromosomes, they are said to be “linked” and DO NOT sort independently.

21 Do “linked” genes sort independently into gametes?
NO! meiosis AaBbCcDdEe

22 Test Cross Show as P_

23 Test Cross Used to determine the unknown genotype
of a dominant phenotype. Cross the dominant phenotype with a recessive phenotype If any of the offspring show recessive trait, the unknown parent was a heterozygote.

24 Incomplete Dominance

25 Incomplete Dominance Heterozygote’s phenotoype is intermediate between dominant and recessive – looks like a blending of the 2. Eg. Pink Four o’clock flowers Genotypic and phenotypic ratios same 1 RR : 2 RW : 1 WW 1 red : 2 pink : 1 white

26 Codominance Codominance is when a heterozygote displays the protein products of both alleles. Roan cow has a mixture of both red and white hairs.

27 Multiple Alleles (I A = IB ) > i
More than 2 different forms of an allele exist. Individual still has just 2. Blood Group alleles: (I A = IB ) > i Blood groups exhibit both codominance and multiple alleles

28 antigen antigen antigens No antigens

29 Pairs 1-22 are autosomes Sex Determination In humans, the 23rd pair of chromosomes determine gender: XX = female XY = male What is the probability of have a son? Daughter?

30 Sex linkage (X linkage)

31 Pedigree Square = male Circle = female Shaded = studied trait
Marriage = horizontal line Offspring = vertical line


33 Sex-Linked Traits This is a pedigree chart showing the inheritance of hemophilia. Does hemophilia affect one gender more often? Why?

34 Sex-Linked Traits Try this test for red/green colorblindness…
What do you see?

35 X-linked traits X and Y are not entirely homologous, X is bigger and carries more genes. Males will have only one allele for traits carried only on the X called X-linked or sex-linked. Some examples of sex-linked traits are hemophilia, colorblindness, and eye color in Drosophila. X-linked disorders will present more in males. Regular chromosomes = autosomes (22 pairs) Pair 23 are sex chromosomes

36 Polygenic Inheritance – many genes affect a single trait – shows continuum. Eg hair color, height, skin color.

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