Presentation on theme: "Mendelian Genetics Start at 2:00."— Presentation transcript:
Mendelian Genetics http://www.youtube.com/watch?v=l1nYsVeLnBE Start at 2:00
Why Peas? Small Easy to grow Short Life Cycle Can Self or Cross pollinate Many easily observed “either/or” traits “True breeding” = Pure breeding, parents will pass on the same traits to the offspring every time.
Monohybrid Cross Cross between parents that differ only in one characteristic First Cross Between Parental (P) Generations Cross one Green Pod plant with one Yellow Pod Plant Results? Offspring (F1 generation) with all yellow pea pods
Second Cross F1 Generation plants Self Fertilize Results? Offspring (F2 generation) with 3:1 ratio yellow to green pod
Dominant Trait = trait visible in the F1 generation (yellow) Recessive Trait =trait that reappeared in the F2 Generation (green)
Principle of Segregation During Meiosis homologous chromosomes will separate into different gametes so that alleles for a given trait will appear in different gametes This allows each parent to contribute one allele for each trait
Allele = alternate form of a gene (yellow and green are 2 different alleles for the pea color gene) Dominant = capital letter (Y = yellow) Recessive = lower case letter (y = green) An individual inherits 2 alleles for each trait (1 from each parent) YyYy Y yy x
Genotype = genetic makeup of an individual (YY, Yy or yy) – Homozygous = both alleles are the same YY or yy – Heterozygous = both alleles are different Yy Phenotype = Appearance of observable characteristics – If both alleles are the same, the trait they code for will be expressed YY = yellow and yy = green – If both alleles are different, the dominant trait will be expressed Yy = Yellow
Punnett Squares Show the expected proportions of possible genotypes in the offspring of 2 individuals. Yellow Pea (YY) x Green Pea (yy) Result 100% offspring genotype will be Yy y y YY Y Y YY yy yy
Principle of Independent Assortment During Gamete formation alleles for one characteristic assort independently of alleles for another characteristic Allows different traits to be inherited separately
Dihybrid Cross Involves two independently assorting traits Cross 2 Heterozygous Round Yellow Pea Plants R = roundr = wrinkled Y = Yellowy = green RrYy x RrYy
Need to identify all the possible allele combinations that can be in the gametes RrYy RYRyrYry
Setup RY Ry ry rY RYRY RYRy RrYY RrYy RRYy RRyyRrYy Rryy RrYYRrYy rrYYrrYy RrYy RryyrrYyrryy
Results Round Yellow Round Green Wrinkled Yellow Wrinkled Green 9 3 3 1 A dihybrid cross between 2 heterozygous organisms always results in a 9:3:3:1 ratio
More Practice! Set up a Dihybrid cross between a heterozygous Yellow Wrinkled pea and a Green heterozygous Round pea plant. Yyrr x yyRr Yr yr yR yr
More Practice! Set up a Dihybrid cross between a heterozygous Yellow Wrinkled pea and a Green heterozygous Round pea plant. Yyrr x yyRr Yr yr yR yr YyRr yyRr YyRr Yyrr yyrr
More Fun with Punnett Squares Incomplete dominance – Occurs when both alleles contribute to the phenotype of a heterozygote to create an intermediate trait – Example: Red flowers x white flowers = pink flower offspring C R C R = Red C W C W = White C R C W = Pink
Co dominance – Both alleles are dominant and are both expressed in the heterozygote – Example: red flower x white flower = red and white flower offspring RR = Red R’R’ = White RR’ = Red and white
What type of dominance is this? A roan cow is the result of a cross between a red cow and a white cow. A roan horse is the result of a cross between a red horse and a white horse. Co dominance!
What type of dominance is this? In human blood types, a homozygote type A person is crossed with a homozygote type B person Result = heterozygote with type AB blood Co-dominance!
Multiple Alleles Some genes have more than 2 allelic forms Example – human blood types A, B, O alleles O is the recessive allele Type A AAAO Type B BBBO Type AB AB Type O OO
Predict the potential blood types of the offspring of a Type A and a Type B person AA x BB AO x BB AA x BO AO x BO 100% AB 50% AB, 50% BO 50% AB, 50% AO 25% AB, 25% AO, 25% BO, 25% OO
X- linked traits Traits whose only gene is carried on the X chromosome. More frequently expressed among males (only 1 X) Examples: colorblindness and hemophilia
X-linked Punnett Square Colorblindness: B = normal vision b= colorblind Mom (not color blind but carries the allele)Dad (not color blind) XBXbXBXb XBYXBY XBXB XbXb XBXB Y XBXB XBXB XBXB XbXb XbXb XBXB Y Y
X- inactivation One X chromosome in females randomly becomes inactivated in each cell (genes won’t be expressed) Barr Bodies = inactivated, condensed X Coloring in Calico cats = black and orange color alleles are on the X chromosome (female cats can inherit one X with orange allele and 1 X with black allele – splotches result from X inactivation) Why don’t we usually see Calico males?
Just to make things a little more interesting… Linkage Genes on the same chromosome are linked and therefore inherited together Don’t follow Mendelian ratios Is there a way to “unlink” genes? – Crossing over!
Epistasis Where one trait prevents the expression of another trait. Example: – If you inherited two genes, one for baldness and another for curly hair, will you see the curly hair gene expressed? – the baldness gene will prevent the expression of the curly hair gene.
Multifactorial Inheritance Polygenic – Traits controlled by more than one gene eye color, height, weight, intelligence, hair color, skin color, metabolism – Don’t follow mendelian ratios of expression Distribution in a population = bell shaped curve