Download Mendel`s Experiments and the Laws of Inheritance

10
Annoncements
• Chapter 9 Online Quiz – Deadline
Tonight
Genetics: Mendel and Beyond
10
Genetics: Mendel and Beyond
• The Foundations of Genetics
• Mendel’s Experiments and the Laws of
Inheritance
• Alleles and Their Interactions
• Gene Interactions
• Genes and Chromosomes
• Sex Determination and Sex-Linked Inheritance
• Non-Nuclear Inheritance
10
Structures Associated with Flowers
http://bio1152.nicerweb.com/doc/class/bio1152/Locked/media/ch30/30_07FlowerStructure_L.jpg
10
Why?
Remember corn in the lab?
Silk + Ear = Female Flower
http://www.agry.purdue.edu/ext/corn/digital.img/2110-08.jpg
“The silks that emerge from the ear shoot are the functional
stigmas of the female flowers. Every potential kernel (ovule) on
an ear develops its own silk that must be pollinated in order for
the ovary to be fertilized and develop into a kernel. Typically, up
to 1000 ovules form per ear, even though we typically harvest only
400 to 600 actual kernels per ear.” -R.L. Nielsen, Purdue University
10
Corn – Male Component
Male flowers on tassel.
Many spikelet male flowers on
tassel. Anthers emerge from
flowers. Pollen released by
anthers.
http://www.agry.purdue.edu/ext/corn/news/
www.hort.purdue.edu/ext/senior/vegetabl/corn3.htm
10
Corn Phenotypes
• Look familiar?
http://www.brookston.org/photoblog/images/20051101074039_indian_corn.jpg
10
The Foundations of Genetics
• Plants have some desirable characteristics for
genetic studies:
 They can be grown in large quantities.
 They produce large numbers of offspring.
 They have relatively short generation times.
 Many have both male and female
reproductive organs, making selffertilization possible.
 It is easy to control which individuals mate.
10
The Foundations of Genetics
• The foundation for the
science of genetics was laid
in 1866, when Gregor
Mendel used varieties of
peas to conduct
experiments on inheritance.
• Mendel’s research was
ignored until the turn of the
twentieth century.
• After meiosis had been
described, several
researchers realized that
chromosomes and meiosis
provided an explanation for
Mendel’s theory.
http://www.weloennig.de/pic/mendel_allein_gefiltert.jpg
Figure 10.1 A Controlled Cross between Two Plants
http://www.dpi.vic.gov.au/
CA25677D007DC87D/L
UbyDesc/Pea+flower.jpg/
$File/Pea+flower.jpg
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel selected varieties of peas that could be studied for
their heritable characters and traits.
• Mendel looked for characters that had well-defined
alternative traits and that were true- breeding, or that
occur through many generations of breeding individuals.
• Mendel developed true-breeding strains to be used as the
parental generation, designated P.
• The progeny from the cross of the P parents are called the
first filial generation, designated F1.
• When F1 individuals are crossed to each other or selffertilized, their progeny are designated F2.
10
Mendel’s Experiments and the Laws of Inheritance
Mendel’s wellorganized plan
allowed him to
observe and record
the traits of each
generation in
sufficient quantity
to explain the
relative proportions
of the kinds of
progeny, i.e. to
predict the traits of
offspring.
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel’s experiment 1:
 A monohybrid cross involves one character (seed
shape) and different traits (smooth (= spherical) or
wrinkled).
 The F1 seeds were all smooth; the wrinkled trait
failed to appear at all.
 Because the smooth trait completely masks the
wrinkled trait when true-breeding plants are
crossed, the smooth trait is considered dominant
and the wrinkled trait recessive.
c. 2004 Sinaur Assoc., Inc.
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel’s experiment 1
continued:
 The F1 generation
was allowed to selfpollinate to produce
F2 seeds.
 In the F2
generation, the ratio
of smooth seeds to
wrinkled seeds was
3:1.
10
Mendel’s Experiments and the Laws of Inheritance
• From these results, Mendel reached several conclusions:
 The units responsible for inheritance are discrete
particles that exist within an organism in pairs and
separate during gamete formation; this is called the
particulate theory. [Therefore, irreversible blending
is not the case.]
 Each pea has two units of inheritance for each
character.
 During production of gametes, only one of the pair for
a given character passes to the gamete.
 When fertilization occurs, the zygote gets one unit
from each parent, restoring the pair.
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel’s units of inheritance are now called
genes; different forms of a gene are called alleles
(e.g., S and s).
• True-breeding individuals have two copies of the
same allele (e.g., SS or ss). They are
homozygous).
• Some smooth-seeded plants are Ss or
heterozygous, although they will not be truebreeding.
• The physical appearance of an organism (e.g.,
smooth seed) is its phenotype; the actual
composition of the organism’s alleles for a gene
(e.g., Ss) is its genotype.
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel’s first law is called the law of
segregation: Each gamete receives one member
of a pair of alleles.
• Mendel arrived at the law of segregation with no
knowledge of meiosis or chromosomes. Today,
the known mechanism of chromosome separation
in meiosis I explains his law of segregation.
Remember Meiosis I?
Separation of homologous
chromosomes! On the
road to 1n.
http://www.sciencecases.org/mitosis
_meiosis/images/meiosis_c.gif
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel verified his hypothesis by performing a test cross.
• A test cross of an individual with a dominant trait with a
true-breeding recessive (homozygous recessive: ss) can
determine the first individual’s genotype [SS
(homozygous) or Ss (heterozygous)].
• If the unknown is heterozygous, approximately half the
progeny will have the dominant trait and half will have the
recessive trait.
• If the unknown is homozygous dominant, all the progeny
will have the dominant trait.
Figure 10.6 Homozygous or Heterozygous?
10
Mendel’s Experiments and the Laws of Inheritance
• Mendel’s second law, the law of independent assortment,
states that alleles of different genes (e.g., Ss and Yy ) assort
into gametes independently of each other.
• To determine this, he used dihybrid crosses, or hybrid
crosses involving additional characters.
• The dihybrid SsYy produces four possible gametes that
have one allele of each gene: SY, Sy, sY, and sy.
• Random fertilization of gametes results in offspring with
phenotypes in a 9:3:3:1 ratio.
10
Meiosis and Independent Assortment
Don’t forget meiosis, which explains both the
law of segregation and the law of independent
assortment.
http://www.sciencecases.org/mitosis_meiosis/images/meiosis_c.gif
10
Formation of Gametes - Genotypes
A key to predicting subsequent offspring [Punnett Squares]
is identifying the genotypes of gametes.
Source: Next slide (Sinaur Associates, Inc.)
Figure 10.7 Independent Assortment
10
Mendel’s Experiments and the Laws of Inheritance
• The basic conventions of probability:
 If an event is certain to happen, its probability
is 1.
 If the event cannot happen, its probability is 0.
 Otherwise the probability is between 0 and 1.
10
Mendel’s Experiments and the Laws of Inheritance
• To determine the probability that two independent
events will both happen, the general rule is to
multiply the probabilities of the individual
events.
• Monohybrid cross probabilities:
 In the example of smooth and wrinkled seeds,
the probability of a gamete being S is ½.
 The probability that an F2 plant will be SS is
1/2 x 1/2 = 1/4
Figure 10.9 Using Probability Calculations in Genetics
10
Mendel’s Experiments and the Laws of Inheritance
• The probability of an event that can occur in two or
more different ways is the sum of the individual
probabilities of those ways.
• The genotype Ss can result from s in the female
gamete (egg) and S in the male gamete (sperm),
or vice versa.
• Thus the probability of heterozygotes in the F2
generation of a monohybrid cross is
1/4 + 1/4 = 1/2
10
Mendel’s Experiments and the Laws of Inheritance
• To calculate the probabilities of the outcomes of
dihybrid crosses, multiply the outcomes from each
of the individual monohybrid components.
• An F1 (dihybrid) cross of SsYy generates 1/4 SS,
1/2 Ss, 1/4 ss, and 1/4 YY, 1/2 Yy, 1/4 yy.
• The probability of the SSYy genotype is the
probability of the SS genotype (1/4), times the
probability of the Yy genotype (1/2), which is 1/8
(1/4 x 1/2 = 1/8).
10
10
Mendel’s Experiments and the Laws of Inheritance
• Because humans cannot be studied using
planned crosses, human geneticists rely on
pedigrees, which show phenotype segregation in
several generations of related individuals.
• Since humans have such small numbers of
offspring, human pedigrees do not show clear
proportions.
• In other words, outcomes for small samples fail to
follow the expected outcomes closely.
10
Mendel’s Experiments and the Laws of Inheritance
• If neither parent has a given phenotype, but it
shows up in their progeny, the trait is recessive
and the parents are heterozygous.
• Half of the children from such a cross will be
carriers (heterozygous for the trait).
• The chance of any one child’s getting the trait is
1/4.
Figure 10.11 Recessive Inheritance
10
Mendel’s Experiments and the Laws of Inheritance
• A pedigree analysis of the dominant allele for
Huntington’s disease shows that:
 Every affected person has an affected parent.
 About half of the offspring of an affected
person are also affected (assuming only one
parent is affected).
 The phenotype occurs equally in both sexes.
Figure 10.10 Pedigree Analysis and Dominant Inheritance
10
Alleles and Their Interactions
• Differences in alleles of genes consist of slight
differences in the DNA sequence at the same
locus, resulting in slightly different protein
products.
• Some alleles are not simply dominant or
recessive. There may be many alleles for a single
character or a single allele may have multiple
phenotypic effects.
10
Alleles and Their Interactions
• Different alleles exist because any gene is subject
to mutation into a stable, heritable new form.
• Alleles can mutate randomly.
• The most common allele in the population is
called the wild type.
• Other alleles, often called mutant alleles, may
produce a phenotype different from that of the
wild-type allele.
• A genetic locus is considered polymorphic if the
wild-type allele has a frequency of less than 99
percent in a population.
10
Alleles and Their Interactions
• A population can have more than two alleles for a
given gene.
• In rabbits, coat color is determined by one gene
with four different alleles. Five different colors
result from the combinations of these alleles.
• Even if more than two alleles exist in a population,
any given individual can have no more than two of
them: one from the mother and one from the
father.
Figure 10.12 Inheritance of Coat Color in Rabbits
10
Alleles and Their Interactions
• Heterozygotes may show an intermediate
phenotype which might seem to support the
blending theory.
• The F2 progeny, however, demonstrate Mendelian
genetics. For self-fertilizing F1 pink individuals the
blending theory would predict all pink F2 progeny,
whereas the F2 progeny actually have a
phenotypic ratio of 1 red:2 pink:1 white.
• This mode of inheritance is called incomplete
dominance.
Figure 10.13 Incomplete Dominance Follows Mendel’s Laws
10
Alleles and Their Interactions
• In codominance, two different alleles for a gene are
both expressed in the heterozygotes.
• In the human ABO blood group system the alleles for
blood type are IA, IB, and IO.
 Two IA, or IA and IO, results in type A.
 Two IB, or IB and IO, results in type B.
 Two IO results in type O.
 IA and IB results in type AB. The alleles are called
codominant.
Figure 10.14 ABO Blood Reactions Are Important in Transfusions
10
Alleles and Their Interactions
• Pleiotropic alleles are single alleles that have
more than one distinguishable phenotypic effect.
• An example is the coloration pattern and crossed
eyes of Siamese cats, which are both caused by
the same allele.
• These unrelated characters are caused by the
same protein produced by the same allele.
10
Gene Interactions
• Epistasis occurs when the alleles of one gene
cover up or alter the expression of alleles of
another gene.
• An example is coat color in mice:
 The B allele produces a banded pattern, called
agouti. The b allele results in unbanded hairs.
 The genotypes BB or Bb are agouti. The
genotype bb is black.
 Another locus determines if any coloration
occurs. The genotypes AA and Aa have color
and aa are albino.
Figure 10.15 Genes May Interact Epistatically
10
Gene Interactions
• In another form of epistasis, two genes are
mutually dependent: The expression of each
depends on the alleles of the other,and they are
called complementary genes.
• For example, two genes code for two different
enzymes that are both required for purple pigment
to be produced in a flower.
• The recessive alleles code for nonfunctional
enzymes. If the plant is homozygous for either a
or b, no purple pigment will form.
10
Gene Interactions
• When two homozygous strains of plants or animals
are crossed, the offspring are often phenotypically
stronger, larger, and more vigorous than either
parent.
• This phenomenon is called hybrid vigor or
heterosis. Hybridization is now a common
agricultural practice used to increase production in
plants.
• A hypothesis called overdominance proposes that
the heterozygous condition in certain genes makes
them superior to either homozygote.
Figure 10.16 Hybrid Vigor in Corn
10
Gene Interactions
• Genotype and environment interact to determine
the phenotype of an organism.
• Variables such as light, temperature, and nutrition
can affect the translation of genotype into
phenotype.
• Penetrance is the proportion of individuals in a
group with a given genotype that express the
corresponding phenotype.
• The expressivity of the genotype is the degree to
which it is expressed in an individual.
10
Gene Interactions
• Complex inherited characteristics are controlled
by groups of several genes, called quantitative
trait loci.
• Each allele intensifies or diminishes the
phenotype.
• Variation is continuous, or quantitative, rather
than qualitative.
• Variation is usually due to two factors: multiple
genes with multiple alleles, and environmental
influences on the expression of these genes.
Figure 10.17 Quantitative Variation
10
Genes and Chromosomes
• How do we determine the order and distance
between the genes that are located on the same
chromosome?
• In 1909, Thomas Hunt Morgan’s lab began its
pioneering studies in Drosophila melanogaster.
10
Genes and Chromosomes
• A test cross between flies that were hybrids for
two alleles and flies that were recessive for both
alleles did not give the expected results of 1:1:1:1.
• Instead, two of the four possible genotypes
occurred at a higher frequency.
• These results make sense if the two loci are on
the same chromosome, and thus their inheritance
is linked.
• Absolute or total linkage of all loci is extremely
rare.
Figure 10.18 Some Alleles Do Not Assort Independently
10
Genes and Chromosomes
• Homologous chromosomes can exchange
corresponding segments during prophase I of
meiosis (crossing over).
• Genes that are close together tend to stay
together.
• The farther apart on the same chromosome
genes are, the more likely they are to separate
during recombination.
Figure 10.19 Crossing Over Results in Genetic Recombination
10
Genes and Chromosomes
• The progeny resulting from crossing over appear
in repeatable proportions, called the recombinant
frequency.
• Recombinant frequencies are greater for loci that
are farther apart on the chromosomes because a
chiasma is more likely to cut between genes that
are far apart.
Figure 10.20 Recombinant Frequencies
10
Genes and Chromosomes
• Recombinant frequencies for many pairs of linked
genes can be used to create genetic maps
showing the arrangement of genes along the
chromosome.
• Scientists now measure distances between genes
in map units.
• One map unit corresponds to a recombination
frequency of 0.01. It also is referred to as a
centimorgan (cM).
Figure 10.21 Steps toward a Genetic Map
10
Sex Determination and Sex-Linked Inheritance
• Sex is determined in different ways in different
species.
• In corn (and peas), which are monoecious, every
diploid adult has both male and female
reproductive structures.
• Other plants and most animals are dioecious:
Some individuals produce only male gametes and
others produce only female gametes.
10
Sex Determination and Sex-Linked Inheritance
• Honeybees: A fertilized egg (2n) gives rise to a
female worker or queen bee, an unfertilized egg (n)
gives rise to a male drone.
• Grasshoppers: Females have two X chromosomes,
males have one. The sperm determines the sex of
the zygote.
• Mammals: females have two X chromosomes,
males have X and Y. Sex of offspring is determined
by the sperm.
10
Sex Determination and Sex-Linked Inheritance
• Disorders can arise from abnormal sex
chromosome constitutions.
• Turner syndrome is characterized by the XO
condition and results in females who physically
are slightly abnormal but mentally normal and
usually sterile.
• The XXY condition, Klinefelter syndrome, results
in males who are taller than average and always
sterile.
10
Sex Determination and Sex-Linked Inheritance
• Some XY individuals lacking a small portion of the
Y chromosome are phenotypically female.
• Some XX individuals with a small piece of the Y
chromosome are male.
• This fragment contains the maleness-determining
gene, named SRY (for sex-determining region on
the Y chromosome).
• The SRY gene codes for a functional protein. If
this protein is present, testes develop; if not,
ovaries develop.
10
Sex Determination and Sex-Linked Inheritance
• A gene on the X chromosome, DAX1, produces
an anti-testis factor.
• The role of SRY gene product in a male is actually
to inhibit the maleness inhibitor encoded by
DAX1.
10
Sex Determination and Sex-Linked Inheritance
• In Drosophila the mechanism is different:
 The males are XY and females XX, but the
ratio of X chromosomes to the autosomal sets
determines sex.
 Two X chromosomes for each diploid set yield
females.
 One X for each diploid set yields males. (XO is
sterile; XY is fertile).
10
Sex Determination and Sex-Linked Inheritance
• Birds, moths, and butterflies have XX males and
XY females. These are called ZZ males and ZW
females to help prevent confusion.
• In these organisms, the egg rather than the sperm
determines the sex of the offspring.
10
Sex Determination and Sex-Linked Inheritance
• The Y chromosome carries very few genes (about
20 are known), whereas the X carries a great
variety of characters.
• Females with XX may be heterozygous for genes
on the X chromosome.
• Males with XY have only one copy of a gene and
are called hemizygous.
• This difference generates a special type of
inheritance called sex-linked inheritance.
Figure 10.23 Eye Color Is a Sex-Linked Trait in Drosophila
10
Sex Determination and Sex-Linked Inheritance
• Pedigree analysis of X-linked recessive phenotypes:
 The phenotype appears much more often in
males than in females.
 A male with the mutation can pass it only to his
daughters.
 Daughters who receive one mutant X are
heterozygous carriers.
 The mutant phenotype can skip a generation if
the mutation is passed from a male to his
daughter and then to her son.
Figure 10.24 Red-Green Color Blindness Is a Sex-Linked Trait in Humans
10
Non-Nuclear Inheritance
• Mitochondria, chloroplasts, and other plastids
possess a small amount of DNA.
• Some of these genes are important for organelle
assembly and function.
• Mitochondria and plastids are passed on by the
mother only, as the egg contains abundant
cytoplasm and organelles.
• A cell is highly polyploid for organelle genes.
• Organelle genes tend to mutate at a faster rate.
Figure 10.4 Mendel’s Explanation of Experiment 1
10
Mendel’s Experiments and the Laws of Inheritance
• Now it is known that a gene is a portion of the
chromosomal DNA that resides at a particular site
(called a locus) and that the gene codes for a
particular function.
• Mendel arrived at the law of segregation with no
knowledge of meiosis or chromosomes. Today,
the known mechanism of chromosome separation
in meiosis I explains his law of segregation.
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 2)
Figure 10. 3 Mendel’s Experiment 1 (Part 1)
Figure 10. 3 Mendel’s Experiment 1 (Part 2)