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Chapter 11 – law of segregation, law of ind. assortment

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1 Chapter 11 – law of segregation, law of ind. assortment
Mendel and the Gene Idea


3 Allele for purple flowers
Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

4 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Could be DD or Dd The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous. Same thing for a 2 trait but testcross would involve the double recessive.

5 LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation
Hypothesis of dependent assortment Hypothesis of independent assortment Sperm 1 YR 1 Yr yR yr Sperm 4 4 1 4 1 4 Eggs 1 YR yr 2 1 2 1 YR Eggs 4 YYRR YYRr YyRR YyRr 1 YR F2 Generation (predicted offspring) 2 YYRR YyRr 1 Yr 4 YYRr YYrr YyRr Yyrr 1 yr 2 YyRr yyrr 1 yR 4 YyRR YyRr yyRR yyRr 3 4 1 4 1 yr 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 3 16 Phenotypic ratio 9:3:3:1

6 Using a dihybrid cross, Mendel developed the law of independent assortment
This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization. Genes located near each other on the same chromosome tend to be inherited together – these are called linked genes and we will do work them!

7 The Spectrum of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In codominance, two dominant alleles affect the phenotype – both expressed! In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties


9 LE 14-12 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc
Fraction of progeny 6/64 1/64

10 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Males = squares, females = circles The trait is shaded in

11 Dominant trait (widow’s peak)
LE 14-14a First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)

12 Recessive trait (attached earlobe)
LE 14-14b First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Attached earlobe Free earlobe Recessive trait (attached earlobe)

13 Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically (look) normal, so they don’t know they have bad genes!

14 Cystic Fibrosis http://www. muschealth. com/video/Default. aspx
Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent - why??? The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes –so lungs full of mucus Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine (so stunted growth)

15 Sickle-Cell Disease – Watch HHMI video
Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include physical weakness, pain, organ damage, and even paralysis

16 Dominantly Inherited Disorders
Some human disorders are due to dominant alleles One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele

17 Blue Fugates of Kentucky: mutant enzyme

18 Marfans – Defective connective tissue

19 Color blindness inheritance – mostly seen in males
Color blindness inheritance – mostly seen in males. How would females get it??

20 Are you color blind? Try to read these!

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