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CHAPTER 5: THE INHERITANCE OF SINGLE-GENE DIFFERENCES
1. What determines how alleles of different genes are transmitted from one
generation to the next?
2. How are characteristics transmitted from one generation to the next?
Terminology (see also Glossary pages 655-681 and web site
http://helios.bto.ed.ac.uk/bto/glossary/)
Ascus – a sac that encloses a tetrad or octad of sexual spores (ascospores)
First division segregation – different alleles go into different nuclei at the first meiotic
division producing an MI division pattern of ascospores
Second division segregation – different alleles go into different nuclei at the second
meiotic division producing an MII division pattern of ascospores
homozygous (= true -breeding): an individual having identical alleles of a gene
heterozygous: an individual having different alleles of a gene
monohybrid: an individual heterozygous at one gene
first filial (F1) generation – the first generation resulting from a controlled cross
between two known parents (P)
second filial (F1) generation – the second generation resulting from a controlled cross
between two known parents (P)
test cross – a cross between an individual of unknown genotype and an individual
homozygous recessive for a particular gene(s)
product rule: the probability that two independent events occur together is equal to the
product of their individual probabilities
sum rule: the probability that either of two mutually exclusive events occurs is equal to
the sum of their individual probabilitiessex chromosome – a chromosome whose
presence is associated with a particular sex
autosome – a chromosome that is not a sex chromosome
sex linkage – the location of a gene on a sex chromosome
hemizygous – a gene present in only one copy in a diploid organism
The segregation of alleles during sexual cell division (Figure 4-20)
1. Consider mitosis in haploid cells, of either genotype A or genotype a:
2. Mitosis in diploid cells, of either genotype AA, aa or Aa
Diploid organisms are classified as :
o homozygous – meaning that they have identical alleles at a given gene (e.g. aa
or AA)
o heterozygous – meaning that they have different alleles at a given gene (e.g.
Aa)
In all cases, mitosis produces daughter cells of identical genotypes to the parent
Consider a diploid meiocyte undergoing meiosis:
Case 1: If a homozgyous meiocyte goes through meiosis, all resulting gametes are of
one type:
Genotype of meiocyte
genotype of
gametes
Case 2: If a heterozygous meiocyte goes through meiosis, the resulting gametes are of
2 types (FIGURE 5-2):
The segregation of chromosomes results in the segregation of alleles
ƒ
To assess the segregation of alleles, you must determine the genotype of the
meiotic products
ƒ
If an allele confers a particular phenotype, the genotype can be inferred from
the phenotype
ƒ
In a haploid life cycle, the haploid products of meiosis (non-transient) can be
scored phenotypically
ƒ
In a diploid life cycle, the haploid products of meiosis are transient and cannot
be scored phentoypically
ƒ
The diploid organism that results from their fusion can be scored
phenotypically
ƒ
Based on the phenotype of the diploid, one can infer the genotype of meiotic
products
i.e. Determining the genotype of meiotic products is simpler and more direct in a
haploid life cycle
ƒ
In both diploid and haploid life cycles, the products of multiple meioses are
usually pooled
ƒ
Therefore, the segregation of alleles within a single meiosis can only be
inferred from the ratio of alleles within the pool
ƒ
Some fungi produce tetrads, in which products of a single meiosis are not
pooled, but rather are maintained in an ascus.
ƒ
In other fungi, the tetrads are linear, meaning that they reflect the segregation
of chromatids during meiosis
The segregation of alleles during meiosis is most directly visible in organisms with a
haploid life cycle which produce linear tetrads (see figure 5-3):
MEIOSIS I - separation of homologous chromosomes
MEIOSIS II - separation of centromere and therefore separation of chromatids
(i.e. formation of four meiotic products)
MITOSIS - duplication of meiotic products to produce eight spores
(FIRST DIVISION SEGREGATION (Figure 5-3)
experiment: a diploid, heterozgyous parent undergoes meiosis
a
a
a
MI
a
A
A
a
a
a
a
M II
a
mitosis
a
A
A
A
A
A
A
A
A
results: a 4:4 ratio in spores (in this case, ratio indicates order as well as number),
different alleles are separated by the first meiotic division = first division segregation
(MI segregation pattern)
conclusion: indicates that before meiosis I, different alleles were on different
homologues (i.e. in their original conformation) and therefore that no recombination
had occurred
SECOND DIVISION SEGREGATION (described in lab manual)
experiment: a diploid, heterozgyous parent undergoes meiosis
a
a
a
a
a
a
MI
A
M II
A
A
A
A
mitosis
A
a
a
A
a
a
A
A
A
results: - a 2:2:2:2 ratio in spores
different alleles are separated in the second meiotic division = second division
segregation (MII division pattern)
conclusion:
-
meiosis I does NOT separate different alleles, therefore different alleles must have
been in the same homologue (chromatids joined by a centromere)
-
suggests that crossover occurred between gene and centromere, so that the
centromere links 2 non-identical chromatids
-
seen as 2:2:2:2 ratio in spores
CALCULATING DISTANCES BETWEEN TWO POINTS ON A CHROMOSOME
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the frequency of crossing over between any two points on a chromosome is directly
proportional to the distance between the two points (more about this in Chapter 6)
-
expressed as an equation:
distance between 2 points = % meiotic products showing recombination between the 2
points
ƒ
In linear octads, the frequency of MII octads is a phenotypic indication that a
recombination has occurred between the gene and the centromere
ƒ
Therefore, can be used to calculate distance between gene and centromere
ƒ
distance between 2 points = % meiotic products showing recombination between the
2 points
ƒ
Within each MII octad, spores = products of meiosis
ƒ
Within these products of meioses, only 1/2 show recombination between gene and
centromere
ƒ
Distance from gene to centromere = 1/2 MII octads
x 100
Total tetrads scored
Question: in the fungus Sordaria, how far is gene A from the centromere?
Experiment: Allow a diploid fungal parent, heterozygous at gene A (Aa) to undergo meiosis
Results: 1000 octads (representing 1000 meioses) are scored, and the following types of octad
seen (MI = first division segregation pattern, MII = second division segregation pattern)
Types of Octads
1 (MI)
2 (MI)
3 (MII)
4 (MII)
5 (MII)
6 (MII)
A
A
A
A
a
a
a
a
a
a
a
a
A
A
A
A
a
a
A
A
a
a
A
A
A
A
a
a
A
A
a
a
A
A
a
a
a
a
A
A
a
a
A
A
A
A
a
a
480
475
13
9
12
11
distance A to centromere
=
1/2 (MII tetrads)
X 100
1000
= 1/2 (13 + 9 + 12 + 11) X 100
1000
= 2.25 map units
conclusion: gene A is 2.25 map units from the centromere
Note: in the above data, 4 different, equally frequent MII patterns arise, depending on how the
spindle attaches to the centromere
INHERITANCE IN ORGANISMS WITH POOLED MEIOTIC PRODUCTS
1. crosses using haploid organisms in which products of meiosis are pooled
-
in organisms with a haploid life cycle, products of meiosis can be scored for
genotype and phenotype in the haploid state
-
therefore, in the cross above, where one parent carries genotype b+ = orange and one
carries genotype b = blue, the products of meiosis will be 1/2 orange and 1/2 blue
2. Crosses using diploid organisms in which products of meiosis are pooled
individual 1
individiual 2
gametes
offspring
-
in the diploid life cycle, the gametes are transient
-
phenotypes can usually only be observed in the diploid stages (in example, parents
and offspring)
-
genotype of the gametes is inferred from the phenotype of the offspring
1) Mendel’s studies on the inheritance of characteristics (box 5-1)
Why use pea plants as a study organism?
• cheap, small, easy to grow
• quick life span, many progeny
• different characteristics available
• cross- or self-pollinate
• diploid
Question: How are characteristics transmitted from one generation to the next?
Methods:
Results:
expected results - blended phenotype
observed results:
purple female x white male = purple F1
white female x purple male = purple F1
- phenotype same as one parent
conclusion: purple phenotype is DOMINANT
Question: is the white characteristic still present (but masked) in the purple F1?
Method:
Results:
Question: What accounts for the 3:1 ratio?
Experiment: Generate F3 progeny from each phenotypic class in the F2.
Results:
Conclusion:
Mendel’s interpretation
1) difference in phenotype (e.g . purple or white) is due to discrete and different
hereditary determinants or factors (factors = genes, different factors = alleles)
2) each plant has 2 copies of the factor (2 alleles) for each character
3) for each factor, one copy is dominant to the other
4) parents pass one copy of each factor to offspring
(one member of each gene segregates randomly into the gametes, so that each gamete
has one allele)
5) union of gametes from each parent is independent of allele the gamete carries
Mendel’s First Law
The two members of a gene pair (alleles) segregate equally into the gametes, so that 1/2
the gametes carry one allele, and 1/2 the gametes carry the other allele
= segregation of homologous chromosomes
Consider the genotypes of Mendel’s peas:
Parents:
White =
Purple =
What gametes will be produced by each parent?
What is the genotype of the F1 (purple) plant?
What are the genotypes in the F2 population, and in what ratio are they found?
Hypothesis: purple F1 is heterozygous, 2/3 of purple plants in F2 are heterozygous
Experiment: Test Cross
- an individual of unknown genotype is crossed to a homozygous recessive individual
- in a test cross, the tester (pp) contributes only recessive gametes, therefore progeny are
a direct indication of the gametes produced by the tested individual
e.g.
Pp X pp
tester
expected gametes from F1=
expected gametes from pp =
Results:
expected progeny:
observed progeny
Conclusions:
SEGREGATION OF SEX-LINKED GENES
-
in many animals and some dioecious plants, sex is determined by the presence of
particular chromosomes, the sex chromosomes
-
remaining, non sex chromosomes are called “autosomes” (Figure 5-6)
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e.g. in Drosophila, there are 4 chromosome pairs,
-
3 pair = autosomal, 1 pair = sex chromosomes
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the two sexes carry a different pair of sex chromosomes
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in most species (e.g. Drosophila and humans), males carry an XY pair, females carry
XX pair
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X and Y chromosomes can pair during meiosis because of the presence of a small
area of homology, the pairing region (Figure 5-7)
-
Remaining regions of the X and Y chromosomes carry different genes, with the Y
chromosome carrying only a small number of genes
female (homogametic sex) produces only X containing gametes
male (heterogametic sex) produces X and Y containing gametes
Meiosis in males:
Division I
Division II
X
X
Sperm Cells
X
X
Y
Y
Y
Y
Sex Linkage
•
gene is on one of the sex chromsomes, usually the X
•
because the gene is on a sex chromsome, the phenotype associated with that gene
will be associated with one sex more frequently than the other
Sex linkage first recognized in experiments with Drosophila - Thomas Morgan (1910)
(Figure 5-8)
characteristic = eye colour
wild type = red eyed = w+
mutant = white eyed = w
Cross 1:
parental: red eyed female x white eyed male
F1: all red eyed
F2: 3 red eyed : 1 white eyed
Cross 2:
Parental: white eyed female x red eyed male
F1: 1 red eyed: 1 white eyed
Conclusions Reciprocal crosses give different results
Hypothesis:
males =
females =
Key points:
-all females get Xw+ from father = all are red
- males receive Y from father that does not carry the w gene and therefore does not
contribute to eye colour phenotype
- phenotype depends on the mother’s contribution
-males get either Xw+ or Xw from mother
1/2 red, 1/2 white
-male is called hemizygous e.g. Xw Y
• has only 1 allele of the w gene, so cannot be homozygous or heterozygous
-whatever allele is present on the male’s X chromosome is expressed phenotypically
Reciprocal cross
parental:
white eyed X
female
(Xw Xw )
gametes:
F1:
only X w
red eyed
male
(Xw+ Y)
Xw+ or Y
1 red eyed : 1 white eyed
(all female) (all male)
Xw+ Xw and X w Y
gametes:
Xw+ and X w
contributes nothing
to eye colour phenotype
Xw and Y
Xw
Xw+ Xw (red female)
Y
Xw+ Y (red male)
Xw
Xw Xw (white female)
Y
Xw Y (white male)
Xw+
Xw
PEDIGREE ANALYSIS
-applies the principles of Mendelian Genetics to humans, where controlled crosses are
not possible, and existing family trees must be used
-random crosses
-small numbers, may not fit expected ratios
Symbols used in pedigrees (figure 5-9):
= male
= female
= unknown gender
= affected individual (female)
= mating between two individuals
children from mating
(in order of birth)
RULES
Autosomal recessive trait (Figure 5-10)
1) phenotype appears with equal frequency in each sex
2) two unaffected individuals produce
3) two affected individuals produce
Assumptions
1) If an allele is rare and recessive, an unaffected individual marrying into a family will
most likely be homozygous for the dominant alelele
E.g. consider a recessive allele a, with a frequency of 1/100
autosomal dominant trait (Figure 5-14)
1) phenotype appears with equal frequency in each sex
2) affected individuals produce
3) affected individuals in every generation
(usually = dominant)
X-linked recessive trait (figures 5-16, 5-17)
1) More males than females with the trait
2) No children of an affected male show the trait, but 1/2 of grandsons will
Xw Y
Xw+ Y
Xw Y
Xw+ Y
Xw+ Xw
Xw Y
Xw+ Xw+
Xw+ Xw
Xw+ Y
Xw+ Y
X-linked dominant trait (figure 5-19)
1) affected males pass the trait on to their daughters but not their sons
2) females married to unaffected males pass the trait on to 1/2 their sons and daughters
SEGREGATION OF GENES IN THE ORGANELLAR GENOMES
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both mitochondria and chloroplasts contain DNA
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during cell fusion in sexual reproduction, male and female cells contribute nuclear
DNA equally, but the female contributes most of the cytoplasm
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cytoplasmic DNA comes only from the mother
-
maternal inheritance (Figures 5-21, 5-22)
-
Chapter 5 problems: 1, 2, 5, 6, 7, 12, 14, 16, 26