1. Mendel and the Gene Idea
Chapter 14
Biology – Campbell • Reece

2. Gregor Mendel 1822-1884 A monk from Austria Taught school
Began breeding garden peas in the abbey garden to study patterns of inheritance

3. Why Pea Plants? They are available in many varieties
They grow quickly and produce many offspring
Have several “either-or” traits
Flower color, flower position, seed color, seed shape, pod shape, pod color, stem length

4. Mendel’s Experiments
Mendel decided to cross plants with opposite traits
He always started with varieties that were true-breeding
The crossing of two true-breeding varieties is called hybridization

5. Mendel’s Experiments P generation – parental generation
F1 generation – first filial generation
F2 generation – second filial generation

6. Mendel’s Experiments

7. Mendel’s Conclusions
Alternative version of genes (different alleles) account for variations in inherited characters.
For each character, an organism inherits two alleles, one from each parent

8. Mendel’s Conclusions
If the two alleles differ, then one, the dominant allele (P), is fully expressed in the organism’s appearance; the other, the recessive allele (p), has no noticeable effect on the organism’s appearance
The two alleles for each character segregate (separate) during gamete production (law of segregation)

9. Genetics Vocabulary
Homozygous – two identical alleles for a trait (PP or pp)
Heterozygous – one of each allele for a trait (Pp)
Phenotype – the physical appearance of the trait (purple flowers)
Genotype – the genetic makeup (PP, pp or Pp)
Testcross – the breeding of a homozygous recessive with an organism of unknown genotype

10. Genotype vs. Phenotype

11. Punnett Squares Used to predict a cross Monohybrid (one trait)
Dihybrid (two traits)
Law of independent assortment

12. Rules of Probability
The rule of multiplication – to predict the chance of two or more independent events occurring simultaneously, multiply the individual probabilities
The rule of addition – the probability of an event that can occur in two or more different ways is the sum of the individual probabilities

13. Rules of Probability
What is the probability of getting at least two recessive traits in the offspring of the following cross: PpYyRr x Ppyyrr?
ppyyRr = ¼ x ½ x ½ = 1/16
ppYyrr = ¼ x ½ x ½ = 1/16
Ppyyrr = ½ x ½ x ½ = 2/16
PPyyrr = ¼ x ½ x ½ = 1/16
ppyyrr = ¼ x ½ x ½ = 1/16
Total = 6/16 or 3/8

14. Incomplete Dominance
Where the F1 hybrids have an appearance somewhere between the phenotypes of the two parent varieties
Red snapdragons x white snapdragons

15. What is a dominant allele?
Complete dominance – one allele is completely dominant over the other (purple vs. white flowers)
Incomplete dominance – a sort of blending of the two traits (red, white, and pink snapdragons
Codominance – both alleles “show” (roan coloring in cattle)
The dominant trait is not necessarily more common in a population

16. Multiple Alleles Three or more alleles determine a trait Blood type:
4 types: A, B, AB, or O
Based on the presence of a certain carbohydrate on the surface of the cell (A or B)
May have one substance (type A or B), both (type AB), or neither (type O)
A and B are codominant

17. Multiple Alleles

18. Pleiotropy The ability of a gene to affect an organism in many ways
For example: sickle-cell disease causes multiple symptoms

19. Epistasis
A gene at one locus (location on the chromosome) alters the phenotypic expression of a gene at a second locus
For example: fur color in mice

20. Polygenic Inheritance
The characters vary in the population along a continuum
An additive effect of two or more genes on a single phenotypic character
For example: skin color & height

21. Pedigrees
An analysis of the results of matings that have already occurred
The information is assembled into a family tree

22. Pedigrees Circle – female Square – male Shaded – has the trait
Unshaded – does not have the trait
Sometimes you will see a half-shaded symbol which represents a carrier of the trait

23. Recessive Disorders
An allele that causes a genetic disorder codes either for a malfunctional protein or for no protein at all
Heterozygotes will be “normal” (they are carriers of the trait)
Homozygous recessive individuals will have the disorder

24. Recessive Disorders
Cystic fibrosis – missing protein for chloride ion transport, results in build up of mucus in the pancreas, lungs, digestive tract, and other organs (most will die before their 5th birthday)
Tay-Sachs – dysfunctional enzyme that fails to break down lipids in the brain (child dies within a few years)

25. Recessive Disorders
Sickle-cell – a substitution of a single amino acid in the hemoglobin protein
When blood oxygen is low, crystals form, causing the blood cells to become sickle shaped
Carriers have increased resistance to malaria

26. Dominant Disorders
If a lethal dominant allele kills an offspring before it can reproduce, the allele will not be passed on
Achondroplasia – a form of dwarfism
Huntington’s disease – a degenerative disease of the nervous system

27. Multifactorial diseases
Diseases that have a genetic component and are influenced by environmental factors.
Include: heart disease, diabetes, cancer, alcoholism, and certain mental illnesses such as schizophrenia and manic-depressive disorder
Hereditary component is often polygenic

28. Fetal Testing
Tests are available for Tay-Sachs, sickle-cell & cystic fibrosis, to determine if the parents are carriers
Amniocentesis – amniotic fluid can be drawn and analyzed to find if there are genetic disorders
Chorionic villus sampling (CVS) – fetal tissue is taken from the placenta

29. Fetal Testing

30. Fetal Testing
Ultrasound – uses sound waves to produce an image of the fetus
Fetoscopy – a needle-thin tube containing a viewing scope and fiber optics is inserted into the uterus (provides a 3-dimensional image)
In 1% of cases, amniocentesis or fetoscopy causes complications

31. Newborn Screening Some genetic disorders can be detected at birth
Phenylketonuria (PKU) – a recessive disorder in which children cannot break down the amino acid phenylalanine
Toxic levels can cause mental retardation
A special diet can prevent retardation