Patterns of Inheritance

Patterns of Inheritance
Chapter 8 Part A

Principles of Inheritance
Inherited traits are passed from parents to offspring Traits are coded in segments of the DNA of chromosomes called genes Through mitosis or meiosis chromosomes are passed to daughter cells Thus traits are passed from parents to offspring

Principles of Inheritance
For sexual reproduction haploid gametes are formed from diploid germ cells Each gamete receives only one of each chromosome pair At fertilization the offspring receive a set of chromosomes from each parent restoring the diploid state The fertilized egg (zygote) contains a complete set of paired chromosomes Thus half the genetic make-up of the offspring comes from the male parent and half from the female parent

Genetic Cross Examine the genetic cross
What trait is different between the two parent mice? What are the two different alleles of the trait? Alleles are different forms of the same gene

What trait is different between the two parent mice? Fur coat color There is a gene that codes for coat color What are the two different alleles of the trait? Alleles are different forms of the same gene Light and dark fur

Which allele is expressed (shows) in all of the offspring? Which allele would be considered dominant? Which allele would be considered recessive?

Which allele is expressed (shows) in all of the offspring? Dark coat color Which allele would be considered dominant? Which allele would be considered recessive? Light coat color

Genetic Cross Genotypes
Assume that the gene for coat color is named with the letter “a” The dark coat color allele will be “A” The light coat color allele will be “a” Remember that each diploid cell contains two of each type of chromosome and thus two of each gene, what are the three possible combinations of alleles that a mouse in this population could have? These combinations are referred to as the organism’s genotype

Assume that the gene for coat color is named with the letter “a” The dark coat color allele will be “A” The light coat color allele will be “a” Remember that each diploid cell contains two of each type of chromosome and thus two of each gene, what are the three possible combinations of alleles that a mouse in this population could have? These combinations are referred to as the organism’s genotype AA two dominant dark coat alleles Aa one dominant dark and one recessive light alleles Aa two recessive light coat alleles

Which of the possible genotypes are homozygous or heterozygous? Homo = the same Hetero = different Homozygous Heterozygous

Which of the possible genotypes are homozygous or heterozygous? Homo = the same Hetero = different Homozygous AA and aa AA = homozygous dominant aa = homozygous recessive Both upper case or both lower case = the same Heterozygous Aa One upper case and one lower case = different

Genetic Cross Phenotypes
The phenotype is the observable traits expressed by the combination of alleles each organism has for a given gene What will the phenotype be for each genotype? AA Aa aa

Genetic Cross Phenotypes What will the phenotype be for each genotype?
AA Dark coat color Has only dominant alleles Aa Has at least one dominant allele aa Light coat color Has only recessive alleles

Genetic Cross Segregation of alleles
During meiosis, homologous chromosomes are separated resulting in a segregation of the two alleles into different gametes The second round of meiosis also separates sister chromatids resulting in four possible gametes with one unduplicated chromosome each Thus offspring receive one allele from each parent

What is the genotype of the parent in the figure? What alleles are carried by the gametes?

What is the genotype of the parent in the figure? Heterozygote or Ff What alleles are carried by the gametes? F, F, f, and f To simplify genetic crosses only one of each duplicate gamete is represented

What is the genotype of the light coat parent? What alleles are carried by the gametes? What is the genotype of the dark coat parent?

What is the genotype of the light coat parent? Homozygous recessive or ff What alleles are carried by the gametes? f and f What is the genotype of the dark coat parent? Homozygous dominant or FF F and F The two alleles segregate and move to separate gametes

Genetic Cross Punnett Square
During fertilization either possible sperm could fuse with either possible ovum A Punnett square can be used to determine all possible gamete combinations and predict what traits the offspring might inherit from the parents

First step: Determine the parent’s genotype Homozygous recessive dad = ff Homozygous dominant mom = FF Second step: Determine the alleles carried by the gametes Remember segregation due to meiosis Homologous pairs of chromosomes are separated (anaphase I). Therefore, during meiosis one “f” will segregate into one gamete, while the other “f” will segregate into the other gamete Dad’s gametes will be f and f Mom’s gametes will be F and F

Third step: Place the letters representing the alleles in the Punnett Square For convenience Dad’s gamete alleles will go in the left hand column Mom’s gamete alleles will go in the right hand column

Fourth step: determine what the possible outcomes are if either of dad’s gametes fuses with either of mom’s eggs Each possible sperm could fuse with each possible ovum or egg

Fifth step: determine the probability of the genotypes and phenotypes Genotype possibilities are FF, Ff, or ff Count up how many out of the four possible offspring have each combination FF:Ff:ff

Fifth step: determine the probability of the genotypes and phenotypes Genotype possibilities are FF, Ff, or ff Count up how many out of the four possible offspring have each combination FF:Ff:ff 0:4:0

Fifth step: determine the probability of the genotypes and phenotypes Phenotype possibilities are Dark fur or light fur Count up how many out of the four possible offspring have each trait Dark:light Dominant alleles are always expressed Recessive alleles are masked by dominant alleles

Fifth step: determine the probability of the genotypes and phenotypes Phenotype possibilities are Dark fur or light fur Count up how many out of the four possible offspring have each trait Dark:light 4:0

What happens if two of the offspring are crossed? 1. What is the genotype of the parents?

What happens if two of the offspring are crossed? 1. What is the genotype of the parents? Ff X Ff

What happens if two of the offspring are crossed? 1. What is the genotype of the parents? Ff X Ff 2. What alleles are carried by the gametes?

What happens if two of the offspring are crossed? 1. What is the genotype of the parents? Ff X Ff 2. What alleles are carried by the gametes? F and f

What happens if two of the offspring are crossed? 3. Place the letters representing the alleles in a Punnett Square

What happens if two of the offspring are crossed? 4. What are the possible outcomes are if either of dad’s gametes fuses with either of mom’s eggs

What happens if two of the offspring are crossed? 5. determine the probability of the genotypes and phenotypes Genotype FF:Ff:ff Phenotype Dark fur:Light fur

What happens if two of the offspring are crossed? 5. determine the probability of the genotypes and phenotypes Genotype FF:Ff:ff 1:2:1 Phenotype Dark fur:Light fur 3:1

Genetic Cross Monohybrid genetic cross (hybridizations)
Examine only one gene with its alleles P generation (parental) Cross two true-breeding individuals that have different traits F1 generation (first filial) Hybrid offspring of the first cross between the P generation Two F1 are crossed with each other F2 generation (second filial) Examining the ratio of characteristics in the P, F1, and F2 generations shows basic patterns of inheritance

Genetic Cross Punnett Square practice of monohybrid crosses

Genetic Cross Independent Assortment
Genes that are on different chromosomes do not influence how they are sorted into gametes They are sorted into gametes independently of each other Demonstrated using a dihybrid cross following the traits of two genes within the same cross

Genetic Cross Independent Assortment: dihybrid cross
A pea plant that produces yellow smooth seeds with a pea plant that produces green wrinkled seeds Information about both genes Color gene Dominant allele: yellow = C Recessive allele: green = c Texture gene Dominant allele: smooth = T Recessive allele: green = t

Cross 1 (P generation) What is the genotype of the parents? Make sure to include two alleles for both genes Dad is yellow and smooth = CCTT Mom is green and wrinkled = cctt

Cross 1 (P generation) What alleles will be carried by the gametes? Make sure to include one allele for each gene

Cross 1 (P generation) What alleles will be carried by the gametes Make sure to include one allele for each gene Dad CCTT  CT, CT, CT, CT Mom cctt  ct, ct, ct, ct

Cross 1 (P generation) Set up the Punnett square Fill in the possible combinations of alleles resulting from fertilization

Cross 1 (P generation) Determine the F1 offspring’s genotype and phenotype ratios Genotype: All are CcTt or heterozygous for both genes Phenotype: All are yellow and smooth

Cross 2 (F1 offspring) Two F1 offspring are crossed What alleles will be carried by the gametes? Make sure to include one allele for each gene

Cross 2 (F1 offspring) Two F1 offspring are crossed What alleles will be carried by the gametes? Make sure to include one allele for each gene Dad CcTt  CT, Ct, cT, ct Mom CcTt  CT, Ct, cT, ct

Cross 2 (F1 offspring) Set up the Punnett square

Cross 2 (F1 offspring) Determine the F2 offspring’s genotype and phenotype ratios

Cross 2 (F1 offspring) Genotypes CCTT CCTt CCtt CcTT CcTt Cctt ccTT ccTt cctt

Cross 2 (F1 offspring) Phenotypes Yellow Smooth Yellow Wrinkled Green Smooth Green Wrinkled Upper case “C” = yellow Upper case “T” = smooth

Cross 2 (F1 offspring) Phenotypes Yellow Smooth 9 Yellow Wrinkled 3 Green Smooth 3 Green Wrinkled 1

Genetic Cross Dihybrid genetic cross
Examine two genes each with two alleles P generation (parental) Cross two true-breeding individuals that have different traits for both genes F1 generation (first filial) Hybrid offspring of the first cross between the P generation Two F1 are crossed with each other F2 generation (second filial) Examining the ratio of characteristics in the P, F1, and F2 generations shows basic patterns of inheritance

Genetic Cross Independent Assortment
Alleles sorted into gametes independently of each other Yellow and smooth were not linked as demonstrated by the presence of Yellow wrinkled and green smooth F2 plants This is the result of random alignment during metaphase I of meiosis I

One of two possible alignments The only other possible alignment
a Chromosome alignments at metaphase I: A a a A A a a B B b b b b B B b The resulting alignments at metaphase II: A A a a A A a a B B b b b b B B B A A B b a a b b A A b B a a B c Possible combinations of alleles in gametes: AB ab Ab aB

Genetic Cross Independent Assortment: Dihybrid practice problems

Gregor Mendel’s Experiments
Used pea plants to show that traits are transmitted faithfully from parents to offspring in specific patterns Pea plants were an excellent model system Self pollinate True breeding Easy to grow Easy to cross one true-breeding variety with another Produce numerous offspring

Used pea plants to show that traits are transmitted faithfully from parents to offspring in specific patterns Examined the ratio of characteristics in the P, F1, and F2 generations of monohybrid and dihybrid crosses

Used pea plants to show that traits are transmitted faithfully from parents to offspring in specific patterns Law of dominance Some alleles are always expressed Cross true-breeding with different traits  all of the offspring express the dominant phenotype Law of segregation Monohybrid cross Anaphase I of meiosis I Law of independent assortment Dihybrid cross Metaphase I of meiosis I

Vocabulary Punnett square Monohybrid cross Dihybrid cross Generations
Genes Alleles Chromosomes Homologous Pair Sister Chromatids Centromere DNA Haploid Diploid Gamete Zygote Germ cells Somatic cells Dominant Recessive Genotype Phenotype Homozygous Heterozygous Punnett square Monohybrid cross Dihybrid cross Generations P, F1 , F2 Gregor Mendel Dominance Segregation Independent assortment

Patterns of Inheritance

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