Final Concepts for Chapter 11Mendelian Genetics • Codominance • Complete dominance • Dihybrid cross • Genotype • Genotypic ratio • Heterozygous • Homozygous • Incomplete dominance • Monohybrid cross • Phenotype • Phenotypic ratio • Probability • Punnett square • Testcross • Expected/predicted results • Actual/observed results • Karyotype • Amniocentesis • Linked genes • Sex-linked disorders • Autosomal disorders Allele Dominant Recessive P-generation F1 generation F2 generation Law of independent assortment Law of segregation Chromosomes Pure breed Trait
Independent Assortment vs. Linked Genes • Mendel did not know about chromosomes when he proposed the Law of Independent Assortment. • The pea traits he studied happened to be located on different chromosomes – so they did assort independently.
Independent Assortment vs. Linked Genes • Question: How many traits do you have? • Question: How many chromosomes (per cell) do you have? • Question: Is it possible to have only one trait per chromosome? • No, lots of genes are carried or linked together on the same chromosome.
Independent Assortment vs. Linked Genes • Do the punnett square for the following cross – assume independent assortment. Cross two heterozygous tall, heterozygous red flowered plants T=tall R=red flower t= short r = white flower
Independent Assortment vs. Linked Genes What is the phenotypic ratio? TtRr x TtRr TR Tr tR tr TR Tr tR tr
Independent Assortment vs. Linked Genes 9:3:3:1 ratio 9 = tall and red 3 = tall and white 3 = short and red 1 = short and white • PROBABILITY: • From this cross, 48 offspring were produced. • How many offspring would you expect to be tall and red? • How many would expect to be tall and white? • How many would you expect to be short and white?
Independent Assortment vs. Linked Genes Now, do the following cross BUT the genes for tallness and red flowers are linked. Cross two heterozygous tall, heterozygous red flowered plants T=tall R=red flower t= short r = white flower
Independent Assortment vs. Linked Genes • Hint T t R r TtRr X TtRr Is it possible to produce a Tr gamete?
Independent Assortment vs. Linked Genes TtRr X TtRr What is the phenotypic ratio? tr TR 3:1 3 = Tall and Red 1 = Short and white TR tr
Independent Assortment vs. Linked Genes • So… out of the 48 offspring, if the genes are linked, how many would be • 1. tall and red? • 2. tall and white? • 3. short and red? • 4. short and white? Answer: tall and red = 36 tall/white = 0 short and white = 12 short/red = 0 EXPECTED RESULTS!
Independent Assortment vs. Linked Genes Is it possible for our Actual Results to show any flowers that are tall/white or short/red? Yes – how? Crossing over
Crossing over occurs in meiosis Pieces of the chromosomes actual switch places.
Codominance – the alleles are equally dominant Roan Cow Human Blood Type
Sex-linked Traits • Traits carried on the X chromosome Fill in the genotypes on the pedigree.
Autosomal disorders • Disorders carried on non-sex chromosomes (first 22 pairs) • Some are autosomal dominant • Huntington’s disease • Most are autosomal recessive • Sickle-cell anemia • Cystic fibrosis
Question: How do you know if the pure bred dog you just paid big bucks for is actually pure? GG? Gg?
Test Cross • Cross using a homozygous recessive individual with a dominant individual to determine if the dominant individual is heterozygous or homozygous dominant (pure) • Why use a homozygous recessive individual?
Test Cross Do the punnett squares for each case: GG x gg Gg x gg
Test Cross • All offspring produced should show the dominant characteristics if the dominant parent is pure (GG) for the trait.
9-1 Mendel’s Legacy • F1 generation are the offspring produced from the original parental group whereas
The dominant factor gets expressed in the individual and the recessive factor can only be expressed when the dominant factor is absent. Ex. Pure Mendelian traits such as Pea Seed Shape S= smooth s = wrinkled
3. An allele is a hereditary factor whereas a gene is a segment of DNA that dictates a trait. Two alleles for every trait: one from mom and one from dad
Multiple Choice • 1. C • 2. A • 3. D • 4. B
SHORT ANSWER • Strain = the body of descendants of a common ancestor, genetic crosses show how “strains” display family traits • Meiosis accounts for both the Law of independent assort. and Law of Segregation because the chromosomes are pulled apart randomly during anaphase 1 and 2 of meiosis
3. F= orange and f = red, then orange is the dominant color • All flowers in the F1 generation would be orange.
4 Critical Thinking: If Mendel studied traits that were linked on the same chromosomes his observations would have led him to very different conclusions. For example, he would not be able to conclude that heredity factors are independent of one another because some would always be displayed together. • EX: round seeds would be produced by red flowering plants only
R r B b r b R B R b r B
9-2 Genetic Crosses • Complete dominance – one allele completely masks the expression of another • Incomplete dominance – both alleles are partially dominant causing an intermediate phenotype (Ex. red and white flower produce a pink flower) • Codominance – both alleles are expressed equally Ex. blood type AB, both A and B antigens are produced
Multiple Choice • 1. B • 2. A 266 short/ total possible of 1,064 • 3. C • 4. D (test cross) • 5. C
Short Answer • 1. A homozygous individual has two of the same alleles for a trait (AA or aa) whereas a heterozygous individual has two different alleles for a trait (Aa) • 2. .25 X 80 = 20 individuals • 3. AA, Aa are the possible genotypes, 100% of offspring will show dominant phenotype
Critical Thinking • 4. One offspring is not sufficient enough to show if the cow is pure bred or not. The larger the sample size the more accurate the conclusion.
WwRr X WwRr • 9 – dominant for both traits • 4 will have same genotype as parents • 1 will be homozygous dominant for both traits • 1 will be homozygous recessive for both traits