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Mendel and the Gene Idea

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1 Mendel and the Gene Idea
Chapter 14 Mendel and the Gene Idea

2 (true-breeding parents) White flowers
LE 14-3 P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation


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

5 LE 14-5_2 3 : 1 P Generation Appearance: Purple flowers PP White
Genetic makeup: Gametes P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 2 P 1 2 p F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

6 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous 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

7 LE 14-7 Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 2 offspring purple and 1 2 offspring white: p p p p P P Pp Pp Pp Pp P P Pp Pp pp pp

8 Mendel identified his second law of inheritance by following two characters at the same time
Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

9 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

10 Using a dihybrid cross, Mendel developed the law of independent assortment
The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation Genes located near each other on the same chromosome tend to be inherited together

11 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 in separate, distinguishable ways In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

12 LE 14-10 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW
F1 Generation Gametes 1 1 2 CR 2 CW Sperm 1 2 CR 1 2 CW Eggs F2 Generation 1 CR 2 CRCR CRCW 1 2 CW CRCW CWCW


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

15 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

16 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)

17 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)

18 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 normal

19 Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

20 Sickle-Cell Disease 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

21 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

22 Huntington’s disease is a degenerative disease of the nervous system – Dominant gene on chromosome 4
The disease has no obvious phenotypic effects until about 35 to 40 years of age

23 LE 14-17a Amniocentesis Amniotic fluid withdrawn A sample of
amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. Fetus Centrifugation Placenta Uterus Cervix Fluid Fetal cells Biochemical tests can be performed immediately on the amniotic fluid or later on the cultured cells. Biochemical tests Several weeks Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Karyotyping

24 LE 14-17b Chorionic villus sampling (CVS) A sample of chorionic villus
tissue can be taken as early as the 8th to 10th week of pregnancy. Fetus Suction tube inserted through cervix Placenta Chorionic villi Fetal cells Biochemical tests Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Several hours Karyotyping

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