Transcript Diapositiva 1 - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Genetica, biologia molecolare ed
evoluzione S
Inheritance, Genes, and
Chromosomes
What Are the Mendelian Laws of Inheritance?
Humans have been deliberately breeding plants and
animals for thousands of years.
Two theories emerged to explain breeding
experiments:
1. Blending inheritance—gametes contain
hereditary determinants that blend in the zygote.
Offspring phenotypes are intermediate.
2. Particulate inheritance—hereditary determinants
are distinct and remain intact at fertilization.
What Are the Mendelian Laws of Inheritance?
Experiments performed by the monk, Gregor
Mendel, supported the particulate theory.
Mendel’s theory of inheritance
was published in 1866 but was
largely ignored until 1900.
He worked with the garden pea, which has both male
and female sex organs and normally self-fertilizes.
Mendel could control pollination and
fertilization by removing the male
organs and manually pollinating the
flowers.
What Are the Mendelian Laws of Inheritance?
Pea plants have many varieties with easily recognized
characteristics.
Character: observable physical feature (e.g., seed
shape)
Trait: form of a character (e.g., round or wrinkled
seeds)
Mendel worked with true-breeding varieties.
What Are the Mendelian Laws of Inheritance?
Mendel developed hypotheses to explain inheritance
of different traits, then designed crossing
experiments to test them.
• He transferred pollen from one plant to another: the
parental generation, P
• The seeds and offspring were the first filial
generation, F1
• In some experiments the F1 plants were allowed to
self-pollinate and produce a second filial
generation, F2
What Are the Mendelian Laws of Inheritance?
Mendel first performed monohybrid crosses:
crossing parental varieties with contrasting traits for a
single character.
•The F1 offspring were not a blend of the two parental
traits. Only one of the traits was present (e.g., round
seeds)— Mendel’s First Law.
•Some F2 had wrinkled seeds. The trait had not
disappeared because of blending.
These results supported the particulate theory.
What Are the Mendelian Laws of Inheritance?
Mendel made monohybrid crosses
for seven traits; all gave similar
results.
The trait that occurred in the F1 and
was more abundant in the F2 was
called dominant, the other
recessive.
In the F2 the ratio of dominant to
recessive traits was about 3:1.
What Are the Mendelian Laws of Inheritance?
Mendel proposed that hereditary determinants
(genes) occur in pairs and segregate from one
another during formation of gametes.
He also proposed that each pea plant has two genes
for each character, one inherited from each parent.
Mendel’s second law— Law of segregation: the two
copies of a gene separate during gamete formation;
each gamete receives only one copy.
What Are the Mendelian Laws of Inheritance?
Different traits arise from different forms of a gene
(now called alleles).
•An organism that is homozygous for a gene has two
alleles that are the same
•An organism that is heterozygous for a gene has
two different alleles. One may be dominant, (e.g.,
round [R]), and the other recessive, (e.g., wrinkled [r])
What Are the Mendelian Laws of Inheritance?
Phenotype is the physical appearance of an
organism.
Genotype is the genetic constitution of the organism.
Mendel proposed that the phenotype is the result of
the genotype.
Allele combinations can be
predicted using a Punnett
square.
What Are the Mendelian Laws of Inheritance?
Genes are now known to be
short sequences of DNA; a DNA
molecule makes up a
chromosome.
Alleles of a gene can separate
during meiosis I.
What Are the Mendelian Laws of Inheritance?
One of Mendel’s hypotheses: there are two possible
allele combinations (RR or Rr) for seeds with the
round phenotype.
He tested this hypothesis by
doing test crosses: F1
individuals are crossed with
homozygous recessive
individuals (rr).
His hypothesis accurately
predicted the results of his
test crosses.
What Are the Mendelian Laws of Inheritance?
Mendel’s third law— Independent assortment:
copies of different genes assort independently.
To test this he crossed true-breeding peas that
differed in 2 characteristics: seed shape and color.
Round, yellow seeds (RRYY)
Wrinkled, green seeds (rryy)
F1 generation is RrYy—all round yellow.
What Are the Mendelian Laws of Inheritance?
Crossing the F1 generation (double heterozygotes) is
a dihybrid cross.
Mendel asked whether, in the gametes produced by
RrYy, the traits would be linked, or segregate
independently.
What Are the Mendelian Laws of Inheritance?
• If linked, gametes would
be RY or ry;
F2 would have three
times more round yellow
than wrinkled green (3:1).
• If independent, gametes
could be RY, ry, Ry, or
rY.
F2 would have nine
different genotypes;
phenotypes would be in
9:3:3:1 ratio.
What Are the Mendelian Laws of Inheritance?
The experiments supported the hypothesis of
independent assortment.
It doesn’t always apply to genes located on the same
chromosome.
But it is correct to say that
chromosomes segregate
independently during
formation of gametes, and
so do any two genes
located on separate
chromosome pairs.
What Are the Mendelian Laws of Inheritance?
One key to Mendel’s success was large sample sizes.
By counting many progeny, he was able to see clear
patterns.
Later, geneticists began using probability calculations
to predict ratios of genotypes and phenotypes, and
statistical techniques to determine whether actual
results matched predictions.
What Are the Mendelian Laws of Inheritance?
Human pedigrees can also show Mendel’s laws.
A pedigree is a family tree showing the occurrence of
phenotypes and alleles.
Humans have small families, and so pedigrees don’t
show the clear proportions that the pea plant
phenotypes did.
What Are the Mendelian Laws of Inheritance?
But pedigrees can be used to determine whether a
rare allele is dominant or recessive.
For rare dominant alleles:
• Every affected person
has an affected parent.
• About half of the
offspring of an affected
parent are also
affected.
What Are the Mendelian Laws of Inheritance?
For rare recessive alleles:
•Affected people can have two parents who are not
affected.
• Only a small proportion of
people are affected: about
¼ of children whose
parents are both
heterozygotes.
• There has usually been a
marriage of relatives
How Do Alleles Interact?
New alleles arise through mutations: stable, inherited
changes in the genetic material.
The allele present in most of the population is called
the wild type. Other alleles are mutant alleles.
Wild-type and mutant alleles reside at the same locus
(specific position on a chromosome).
A genetic locus is polymorphic if the wild-type allele
is present less than 99% of the time.
How Do Alleles Interact?
Any one individual has 2 alleles at a locus, but there
may be many alleles in a population.
Multiple alleles often show a hierarchy of dominance.
How Do Alleles Interact?
Some alleles are neither dominant nor recessive—a
heterozygote has an intermediate phenotype:
incomplete dominance.
In the F2, the
original
phenotypes
reappear, the
alleles have not
“blended.”
How Do Alleles Interact?
Codominance: two alleles produce phenotypes that
are both present in the heterozygote.
The ABO blood group system results from three
different alleles that encode an enzyme that adds
specific groups to oligosaccharides on red blood cell
surfaces.
The three alleles, IA, IB, and IO produce
different versions of the enzyme.
How Do Alleles Interact?
Pleiotropic: one allele has multiple phenotypic
effects.
The heritable human disease phenylketonuria (PKU)
results from a mutation in the gene for a liver enzyme
that converts the amino acid phenylalanine to tyrosine.
Phenylalanine builds up to toxic levels, and affects
development.
The mutated allele is pleiotropic: it results in mental
retardation, and reduced hair and skin pigmentation.
How Do Genes Interact?
Epistasis: phenotypic expression of one gene is
influenced by another gene.
Coat color in Labrador retrievers:
for alleles B (black) and b
(brown) to be expressed, allele
E (pigment deposition) must be
expressed.
An ee dog is yellow regardless
of which B alleles are present. E
is said to be epistatic to B.
How Do Genes Interact?
Inbreeding: mating among close relatives; can result
in offspring of low quality.
Close relatives tend to have the same recessive
alleles.
A cross between two different true-breeding
homozygotes can result in offspring with stronger,
larger phenotypes.
Called “hybrid vigor” or
heterosis.
How Do Genes Interact?
The pea characters Mendel studies were discrete and
qualitative.
For more complex characters, phenotypes vary
continuously over a range—quantitative, or
continuous, variation.
Quantitative variation is usually due to both genes and
environment.
Genes that determine these complex characters:
quantitative trait loci.
What Is the Relationship between Genes and Chromosomes?
In 1909, Thomas Hunt Morgan and students at
Columbia University pioneered the study of the fruit fly
Drosophila melanogaster.
Some crosses performed with Drosophila did not yield
expected ratios according to the law of independent
assortment.
Some genes were inherited
together; the two loci were on the
same chromosome, or linked.
All of the loci on a chromosome
form a linkage group.
What Is the Relationship between Genes and Chromosomes?
Absolute linkage is rare—genes on the same
chromosome do sometimes separate.
Genes may recombine during
prophase I of meiosis by crossing
over.
Chromosomes exchange
corresponding segments. The
exchange involves two chromatids in
the tetrad; both chromatids become
recombinant.
Recombinant offspring phenotypes
appear in recombinant
frequencies.
What Is the Relationship between Genes and Chromosomes?
Recombinant frequencies can be used to infer the
location of genes on a chromosome, and make
genetic maps.
What Is the Relationship between Genes and Chromosomes?
Corn: each adult produces both male and female
gametes—monoecious.
Some plants and most animals are dioecious—male
and female gametes produced by different individuals.
In many animals, sex is determined by a single pair of
sex chromosomes which differ from one another.
Both sexes have two copies of all other
chromosomes, called autosomes.
What Is the Relationship between Genes and Chromosomes?
The Y fragment in both cases contains SRY (sexdetermining region on the Y chromosome).
Primary sex determination:
If SRY protein is present, the embryo develops testes.
If there is no SRY, the embryo develops ovaries.
A gene on the X chromosome, DAX1, produces an
anti-testis factor.
In males, SRY inhibits the DAX1 anti-testis factor.
In females (who lack SRY), DAX1 functions to inhibit
maleness.
What Is the Relationship between Genes and Chromosomes?
Secondary sex characteristics: the outward
manifestations of sex.
The gonads produce hormones (testosterone and
estrogen) that control the development of these
characteristics.
What Is the Relationship between Genes and Chromosomes?
Sex chromosome abnormalities can result from
nondisjunction in meiosis—aneuploidy (abnormal
number of chromosomes).
• XO—Turner syndrome
• XXY—Klinefelter syndrome
• Some women are XY but lack a small piece of the Y
chromosome.
• Some men are XX but a small piece of the Y
chromosome is attached to another chromosome.
What Is the Relationship between Genes and Chromosomes?
A single copy of a gene is called hemizygous.
Sex-linked inheritance: inheritance of a gene carried
on a sex chromosome.
In mammals the X chromosome is larger and carries
more genes than the Y, so sex-linked genes are
usually on the X chromosome.
How Do Prokaryotes Transmit Genes?
Bacteria exchange genes by conjugation; requires
physical contact between cells.
A sex pilus extends from one cell to another, and
brings them together.
Genetic material can pass through a thin cytoplasmic
bridge called the conjugation tube.
DNA passes from a donor cell to a
recipient cell.
The donor DNA lines up with the
recipient’s DNA and crossing over can
occur, changing the recipient’s genetic
makeup.
How Do Prokaryotes Transmit Genes?
Bacteria also have plasmids—small circular
chromosomes.
Plasmid genes fall into these categories:
•Unusual metabolic functions
•Antibiotic resistance genes (R factors)
•Genes for making a sex pilus
Plasmids can move between the cells
during conjugation.
Plasmids can replicate independently
of the main chromosome, or be
integrated into the main chromosome.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved