1. CHAPTER 9 Patterns of Inheritance
Overview:
Mendel’s Laws
Variations of Mendel’s Laws
Chromosomes
Sex linked genes

2. Purebreds and Mutts — A Difference of Heredity
Genetics is the science of heredity
These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups
Purebreds often suffer from serious genetic defects

3. The parents of these puppies were a mixture of different breeds
Their behavior and appearance is more varied as a result of their diverse genetic inheritance

4. The science of genetics has ancient roots
MENDEL’S LAWS
The science of genetics has ancient roots
The science of heredity dates back to ancient attempts at selective breeding
Until the 20th century, however, many biologists erroneously believed that
characteristics acquired during lifetime could be passed on
characteristics of both parents blended irreversibly in their offspring

5. Experimental genetics began in an abbey garden
Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants
Was the first person to analyze patterns of inheritance
Deduced the fundamental principles of genetics

6. Mendel studied garden peas
These plant are easily manipulated
These plants can self-fertilize

7. Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation
This illustration shows his technique for cross-fertilization

8. He also created true-breeding varieties of plants
Mendel then crossed two different true-breeding varieties, creating hybrids

9. Mendel studied seven pea characteristics
He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity

10. One characteristic comes from each parent
Mendel’s principle of segregation describes the inheritance of a single characteristic
From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic
One characteristic comes from each parent
A monohybrid cross is a cross between parent plants that differ in only one characteristic

11. Mendel’s principle of segregation
Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes at fertilization creates allele pairs again
Allele: Any one of the alternative forms of a given gene (e.g. the ABO gene has three major alleles: A, B and O alleles). Alternative forms of a gene (alleles).

12. A sperm or egg carries only one allele of each pair
The pairs of alleles separate when gametes form
This process describes Mendel’s law of segregation
Alleles can be dominant or recessive
An explanation of Mendel’s results, including a Punnett square

13. Homologous chromosomes bear the two alleles for each characteristic
Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes

14. Genetic Alleles and Homologous Chromosomes
Have genes at specific loci
Have alleles of a gene at the same locus

15. Homozygous Heterozygous
When an organism has identical alleles for a gene
Heterozygous
When an organism has different alleles for a gene

16. The principle of independent assortment is revealed by tracking two characteristics at once
By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation
This is known as the principle of independent assortment

17. Mendel’s Principle of Independent Assortment
Two hypotheses for gene assortment in a dihybrid cross
Dependent assortment
Independent assortment

18. Mendel’s principle of independent assortment
Each pair of alleles segregates independently of the other pairs during gamete formation

19. Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between
An individual of unknown genotype and
A homozygous recessive individual

20. Mendel’s principles reflect the rules of probability
Inheritance follows the rules of probability
The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring

21. Connection: Genetic traits in humans can be tracked through family pedigrees
The inheritance of many human traits follows Mendel’s principles and the rules of probability

22. Connection: Many inherited disorders in humans are controlled by a single gene
Most such disorders are caused by autosomal recessive alleles
Examples: cystic fibrosis, sickle-cell disease

23. A few are caused by dominant alleles
Examples: achondroplasia, Huntington’s disease

24. Connection: Fetal testing can spot many inherited disorders early in pregnancy
Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions
Fetal cells can be obtained through amniocentesis

25. VARIATIONS ON MENDEL’S PRINCIPLES
The relationship of genotype to phenotype is rarely simple
Mendel’s principles are valid for all sexually reproducing species
However, often the genotype does not dictate the phenotype in the simple way his principles describe
Phenotype
An organism’s physical traits
Genotype
An organism’s genetic makeup

26. BEYOND MENDEL
Some patterns of genetic inheritance are not explained by Mendel’s principles

27. Incomplete Dominance in Plants and People
In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents

28. Many genes have more than two alleles in the population
In a population, multiple alleles often exist for a characteristic
The three alleles for ABO blood type in humans is an example

29. A single gene may affect many phenotypic characteristics
A single gene may affect phenotype in many ways
This is called pleiotropy
The allele for sickle-cell disease is an example

30. Connection: Genetic testing can detect disease-causing alleles
Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring

31. A single characteristic may be influenced by many genes
This situation creates a continuum of phenotypes
Example: skin color

32. Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotype

33. THE CHROMOSOMAL BASIS OF INHERITANCE
Chromosome behavior accounts for Mendel’s principles
Genes are located on chromosomes
Their behavior during meiosis accounts for inheritance patterns

34. Genes on the same chromosome tend to be inherited together
Certain genes are linked
They tend to be inherited together because they reside close together on the same chromosome

35. This inheritance pattern was later explained by linked genes, which are
Genes located on the same chromosome
Genes that are typically inherited together

36. Crossing over produces new combinations of alleles
This produces gametes with recombinant chromosomes
The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

37. Geneticists use crossover data to map genes
Crossing over is more likely to occur between genes that are farther apart
Recombination frequencies can be used to map the relative positions of genes on chromosomes

38. SEX CHROMOSOMES AND SEX-LINKED GENES
Chromosomes determine sex in many species
A human male has one X chromosome and one Y chromosome
A human female has two X chromosomes
Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

39. Sex-linked genes exhibit a unique pattern of inheritance
All genes on the sex chromosomes are said to be sex-linked
In many organisms, the X chromosome carries many genes unrelated to sex
Fruit fly eye color is a sex-linked characteristic

40. Their inheritance pattern reflects the fact that males have one X chromosome and females have two
These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait

41. Connection: Sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles
Examples: hemophilia, red-green color blindness
These are mostly seen in males
A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected