CHAPTER 9 Patterns of Inheritance

1
CHAPTER 9Patterns of Inheritance
Overview Mendels Laws Variations of Mendels
Laws Chromosomes Sex linked genes
2
Purebreds and Mutts A Difference of Heredity

• Genetics is the science of heredity
• These black Labrador puppies are purebredtheir
  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
MENDELS LAWS

• 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 Mendels
  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
Mendels 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

• Mendels 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 Mendels law of
  segregation
• Alleles can be dominant or recessive

• An explanation of Mendels 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

• Homologous chromosomes

• Have genes at specific loci
• Have alleles of a gene at the same locus

15

• Homozygous

• 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
Mendels Principle of Independent Assortment

• Two hypotheses for gene assortment in a dihybrid
  cross

• Dependent assortment
• Independent assortment

18

• Mendels 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
Mendels 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
  Mendels 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, Huntingtons 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 MENDELS PRINCIPLES
The relationship of genotype to phenotype is
rarely simple

• Mendels 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 organisms physical traits
• Genotype
• An organisms genetic makeup

26
BEYOND MENDEL

• Some patterns of genetic inheritance are not
  explained by Mendels 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 Mendels
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