1. Modifications to Mendelian Inheritance
I. Allelic, Genic, and Environmental Interactions

2. Modifications to Mendelian Inheritance
I. Allelic, Genic, and Environmental Interactions
A. Overview:
The effect of a gene is influenced at three levels:
- Intralocular (effects of other alleles at this locus)
- Interlocular (effects of other genes at other loci)
- Environmental (the effect of the environment on
determining the effect of a gene on the phenotype)
A a
GENOME
Environment
PHENOTYPE

3. A a I. Allelic, Genic, and Environmental Interactions A. Overview:
B. Intralocular Interactions
A a

4. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
- The presence of one allele is enough to cause the complete expression of a given phenotype.

5. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
- The heterozygote expresses a phenotype between or intermediate to the phenotypes of the homozygotes.

6. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely; the heterozygote does not have an intermediate phenotype, it has BOTH phenotypes.
ABO Blood Type:
A = ‘A’ surface antigens
B = ‘B’ surface antigens
O = no surface antigen from this locus
Phenotype Genotypes
A AA, AO
B BB, BO
O OO
AB AB
codominance
AB Phenotype

7. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
– the heterozygote expresses a phenotype MORE EXTREME than either homozygote
TEMP
Enzyme Activity
“T”
“t”
TT = tall (grows best in warm conditions)
tt = short (grows best in cool conditions)
Tt = Very Tall (has both alleles and so grows optimally in cool and warm conditions)

8. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
- Essential genes: many proteins are required for life. “Loss-of-function” alleles may not affect heterozygotes, but in homozygotes may result in the death of the zygote, embryo, or adult – depending on when they should be expressed during development.

9. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
- Essential genes: many proteins are required for life. “Loss-of-function” alleles may not affect heterozygotes, but in homozygotes may result in the death of the zygote, embryo, or adult – depending on when they should be expressed during development.
Recessive Lethals:
Aa x Aa
- 25% reduction in
number of offspring A a AA Aa aa
Self-crossing the survivors shows that 1/3 show no reduction in offspring number (AA), while 2/3 show the 25% reduction in number (Aa)
Why haven’t they been weeded out of the population by selection?

10. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Sometimes, the heterozygote has a different phenotype than the homozygote. The phenotypic effect can be ‘dominant’ while the lethal effect is recessive. AY exerts a dominant effect on coat color (expressed in the heterozygote), but is lethal ONLY in the homozygous condition (recessive lethality).
Also an example of pleiotropy – one gene affecting >1 trait.

11. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Conditional Lethality:
In this case, the expression of lethality only occurs under specific conditions. Favism is caused by a mutation in the gene that codes for the enzyme glucose-6-phosphate dehydrogenase. When afflicted individuals eat fava beans, their red blood cells rupture and clog capillaries, resulting in anemia and death.
It is the most common enzyme defect in humans.
Why not selected against?

12. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Conditional Lethality:
In this case, the expression of lethality only occurs under specific conditions. Favism is caused by a mutation in the gene that codes for the enzyme glucose-6-phosphate dehydrogenase. When afflicted individuals eat fava beans, their red blood cells rupture and clog capillaries, resulting in anemia and death.
It is the most common enzyme defect in humans.
Why not selected against?
Balanced selection… provides some protection against malaria.

13. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Dominant Lethal
Why aren’t they weeded out of the population?

14. HC is a neurodegenerative disorder caused by an autosomal lethal dominant allele.
The fishing villages around Lake Maracaibo in Venezuela have the highest incidence of Huntington’s Chorea in the world, approaching 50% in some communities.
Dr. Nancy Wexler's work
The gene was mapped to chromosome 4, and the HC allele was caused by a repeated sequence of over 35 “CAG’s”. Dr. Nancy Wexler found homozygotes in Maracaibo and described it as the first truly dominant human disease (most are incompletely dominant and cause death in the homozygous condition).

15. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethality:
6. Multiple Alleles:
- not really an interaction, but a departure from simple Mendelian postulates.
- and VERY important as a source of variation
# Alleles at the Locus
# Genotypes Possible
1 (A)
1 (AA)
2 (A, a)
3 (AA, Aa, aa)
3 (A, a, A’)
6 (AA, Aa, aa, A’A’, A’A, A’a) 4 10 5 15

16. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethality:
6. Multiple Alleles:
7. Penetrance and Expressivity:
- Penetrance: the percentage of individuals with a given genotype that actually EXPRESS the associated phenotype. (Because of environment or other genes)
- Expressivity: The DEGREE to which an individual expresses its genetically determined trait. The degree of “eyeless” expression in Drosophila is affected by genetic background and environment.

17. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
- Summary and Implications:
populations can harbor extraordinary genetic variation at each locus, and these alleles can interact in myriad ways to produce complex and variable phenotypes.
Consider this cross: AaBbCcDd x AABbCcDD
Assume:
The genes assort independently A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the offspring?

18. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
- Summary and Implications:
populations can harbor extraordinary genetic variation at each locus, and these alleles can interact in myriad ways to produce complex and variable phenotypes.
Consider this cross: AaBbCcDd x AABbCcDD
Assume:
The genes assort independently A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the offspring?
A B C D
x x x 1 = 18
If they had all exhibited complete dominance, there would have been only:
x x x 1 = 4
So the variety of allelic interactions that are possible increases phenotypic variation multiplicatively. In a population with many alleles at each locus, there is an nearly limitless amount of phenotypic variability.

19. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
The phenotype can be affected by more than one gene.

20. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
There may be several genes that produce the same protein product; and the phenotype is the ADDITIVE sum of these multiple genes.
Creates continuously variable traits.
So here, both genes A and B produce the same pigment. The double homozygote AABB produces 4 ‘doses’ of pigment and is very dark. It also means that there are more ‘intermediate gradations’ that are possible.

21. Genotype at H
Genotype at A,B,O
Phenotype H- A- A B- B OO O AB hh
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
one gene masks/modifies the expression at another locus; the phenotype in the A,B,O blood group system can be affected by the genotype at the fucosyl transferase locus.
This locus makes the ‘H substance’ to which the sugar groups are added to make the A and B surface antigens.
A non-function ‘h’ gene makes a non-functional foundation and sugar groups can’t be added – resulting in O blood regardless of the genotype at the A,B,O locus. This ‘O’ is called the ‘Bombay Phenotype’ – after a woman from Bombay (Mumbai) in which it was first described.

22. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
So, what are the phenotypic ratios from this cross:
HhAO x HhBO?

23. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
So, what are the phenotypic ratios from this cross:
HhAO x HhBO?
Well, assume they are inherited independently.
AT H: ¾ H: ¼ h
At A,B,O: ¼ A : ¼ O: ¼ B : ¼ AB
So, the ¼ that is h is O type blood, regardless.
Then, we have:
¾ H x ¼ A = 3/16 A
¾ H x ¼ O = 3/16 O (+ 4/16 above)
¾ H x ¼ B = 3/16 B
¾ H x ¼ AB = 3/16 AB
Phenotypic Ratios: 3/16 A : 3/16 B : 3/16 AB : 7/16 O = 16/16 (check!)

24. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
Process:
enzyme enzyme 2
Precursor precursor product
(pigment)

25. Process:
enzyme enzyme 2
Precursor precursor product
(pigment)
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.
Strain 1:
enzyme enzyme 2
Precursor precursor no product
(white)
Strain 2:
enzyme enzyme 2
Precursor precursor no product
(white)

26. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.
So there must be a dominant gene at both loci to produce color.
Genotype Phenotype
aaB- white
aabb white
A-bb white
A-B- pigment
So, what’s the phenotypic ratio from a cross:
AaBb x AaBb ?

27. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.
So there must be a dominant gene at both loci to produce color.
Genotype Phenotype
aaB- white
aabb white
A-bb white
A-B- pigment
So, what’s the phenotypic ratio from a cross:
AaBb x AaBb ?
9/16 pigment (A-B-), 7/16 white

28. C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.
So there must be a dominant gene at both loci to produce color.
Indeed, by mating two strains together, we can determine whether the mutation is the result of different alleles at the same locus, or different GENES acting on one PATHWAY. This is called a complementation test.

29. Consider two strains that are wingless
Consider two strains that are wingless. Do these strains have different “loss of function” mutations in the same gene, or mutations in different genes involved in the same process (wing development)?

30. Genotype Phenotype
rrpp single
R-pp rose
rrP- pea
R-P- Walnut
C. Interlocular Interactions
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
-example #3: Novel Phenotypes.
Comb shape in chickens is governed by 2 interacting genes that independently produce “Rose” or “Pea” combs, but together produce something completely different (walnut).

31. Genotype Phenotype
aabb long
A-bb sphere
aaB- sphere
A-B- disc
C. Interlocular Interactions
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.
-example #3: Novel Phenotypes.
Comb shape in chickens is governed by 2 interacting genes that independently produce “Rose” or “Pea” combs, but together produce something completely different (walnut).
Fruit shape in summer squash is influnced by two interacting loci, also.

32. C. Interlocular Interactions 1. Quantitative (Polygenic) Traits:
2. Epistasis:
In all of these cases, the observed ratios are modifications of the basic Mendelian Ratios.
A-B-

33. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects:
The environment - the cellular environment as well as the environment outside the organism - can influence whether and how an allele is expressed ,and the effect it has. Typically, what we see is that a gene is only expressed under certain conditions. This is called “conditional expression”, like conditional lethality discussed earlier.

34. D. Environmental Effects:
1. TEMPERATURE
- Siamese cats and Himalayan rabbits – dark feet and ears, where temps are slightly cooler. Their pigment enzymes function at cool temps. If raised at cooler temps, they are more pigmented… even looking like a genetically melanic form. When this happens in rabbits, the Himalayan genotype is expressing a phenocopy – copying the phenotype of another genotype.
- Arctic fox, hares – their pigment genes function at high temps and are responsible for a change in coat color in spring and fall, and a change back to white in fall and winter.
So, these genes have conditional expression.

35. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
- people have genetically different sensitivities to different toxins. Certain genes are associated with higher rates of certain types of cancer, for example. However, they are not ‘deterministic’… their effects must be activated by some environmental variable.
PKU = phenylketonuria… genetic inability to convert phenylalanine to tyrosine. Phenylalanine can build up and is toxic to nerve cells. Single gene recessive disorder.
But if a homozygote recessive eats a diet low in phenylalanine, no negative consequences develop. So, the genetic predisposition to express the disorder is influenced by the environment. These genes have conditional expression.

36. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
3. THE GENETIC ENVIRONMENT – “EPIGENETICS”
Epigenetics is the study of the heritable changes in the expression of genes unrelated to changes in the actual DNA sequence of the genes.
Heritable changes due to different patterns in gene regulation
- within an organism: tissue specialization, resulting in heritable changes in a cell line through mitotic generations, creating different cell and tissue types (even though they all have the same DNA).
- between organisms: regulatory differences between monozygotic twins make them phenotypically different, even though they are genetically identical.

37. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
3. THE GENETIC ENVIRONMENT – “EPIGENETICS”
1. Position Effects
- the effect of a gene may be influenced by WHERE it is in the genome – and its immediate neighbors. Genes can change position due to a variety of mutations. It may be placed next to or into densely coiled ‘heterochromatin’ and be turned off. Or, it may be placed next to an ‘enhancer’ and ‘up-regulated’ (turned on more).
Position effect variegation in eye pigmentation in Drosophila.

38. 1. Position Effects
Can be transcribed – ‘on’
Can’t be transcribed – ‘off’

39. 1. Position Effects
Human Diseases Caused by Position Effects
Aniridia (malformed iris in the eye)
X-linked deafness
B-Thalassemia
SRY sex reversal
Split hand and foot malformation

40. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
3. THE GENETIC ENVIRONMENT – “EPIGENETICS”
1. Position Effects
2. Imprinting
- The effect of a gene can depend on the parent it is inherited from. During gamete formation, both sexes selectively ‘imprint’ certain genes. This amounts to turning them on or off, regardless of their previous state in the organisms own body.

41. 2. Imprinting
- “insulin-like growth factor 2” (igf-2) is a protein, produced in mammalian liver cells, that circulates in the blood. It stimulates cell growth and mitosis during fetal development. Mutation / inactivation causes small size at birth.

42. 2. Imprinting
- “insulin-like growth factor 2” (igf-2) is a protein, produced in mammalian liver cells, that circulates in the blood. It stimulates cell growth and mitosis during fetal development. Mutation / inactivation causes small size at birth.
Mutant male x normal female
ss SS
Normal male x mutant female SS ss Ss Ss
All heterozygous, but all small
All heterozygous, but all normal
RECIPROCAL CROSSES YIELD DIFFERENT RESULTS –
THE SEX OF THE PARENT EXPRESSING NORMAL SIZE “MATTERS” FOR THE PHENOTYPE OF THE OFFSPRING. If the gene for normal growth comes from males, it is ON, if it comes from females, it is OFF.

43. Confirmed in the F1 X F1 CROSS:
2. Imprinting
Confirmed in the F1 X F1 CROSS:
Activity is not a function of the animal’s own sex, it is a function of the sex of the parent that gave the gene.
This example is a bit misleading… it equates ‘imprinting’ only with the female turning the gene off. But imprinting is regulation that can also turn a gene on.

44. Confirmed in the F1 X F1 CROSS:
Small
Small
2. Imprinting
Confirmed in the F1 X F1 CROSS:
‘off’ in this male
‘off’ in this female
The same pattern occurs when SMALL heterozygotes are mated together. ‘Imprinting’ occurs during gamete formation, with males turning ON the gene in sperm (although it was off in their own cells), and females turning OFF the gene in eggs (which was also ‘off’ in their own cells).
Imprinted ‘on’

45. 2. Imprinting
- why?
Haig ‘Parental Conflict’ Hypothesis:
- selection will favor males that increase the growth of their offspring, so they should pass on igf-2 genes that are ON.
- but the female actually provides the energy for embryonic growth, and the energetic demands of maximal embryonic growth will reduce her survival and subsequent reproduction. Her most adaptive reproductive strategy is to reduce the growth of embryos to a reasonable level that doesn’t threaten her own survival.
- she turns OFF her igf-2 genes that she passes to her offspring.
Different selective pressures on males and females, selecting for different behaviors, different patterns of energy allocation, and different patterns of gene activation.

46. 2. Imprinting
- why?
Haig ‘Parental Conflict’ Hypothesis:
- selection will favor males that increase the growth of their offspring, so they should pass on igf-2 genes that are ON.
- but the female actually provides the energy for embryonic growth, and the energetic demands of maximal embryonic growth will reduce her survival and subsequent reproduction. Her most adaptive reproductive strategy is to reduce the growth of embryos to a reasonable level that doesn’t threaten her own survival.
- she turns OFF her igf-2 genes that she passes to her offspring.
A Test: The igf-2 receptor protein in cell membranes.
- this receptor degrades the igf-2 protein, reducing its effect on growth.
- Haig and Graham predicted this should be imprinted as well, and OFF when passed from males and ON when passed from females, based on the ‘parental conflict’ hypothesis.
- when tested, it had this pattern of imprinting – supporting their hypothesis.

47. 2. Imprinting
- why?
- how?
- methyl groups are added to cytosines in CG sequences in the promoter (“CpG islands”), interrupting the binding of the RNA polymerase, stopping transcription (turning the gene OFF).

48. 2. Imprinting
- why?
- how?
- are they common and important?
rare - ~80 have been identified in 30 years, in animals and plants.

49. 2. Imprinting
- why?
- how?
- are they common and important?
rare - ~80 have been identified in 30 years, in animals and plants.
But in humans, several diseases are caused by the failure to inherit correctly imprinted genes.

50. Prader-Willi Syndrome:
- hunger and weight gain
- cognitive disability
- reduced sex organs
- decreased muscle tone

51. Prader-Willi Syndrome:
- hunger and weight gain
- cognitive disability
- reduced sex organs
- decreased muscle tone
Caused by deletion in the 15q11-13 region in the father. This region is methylated (‘off’) in eggs, and should be ON in sperm. Deletion in the father results in embryos that lack one gene (from father) and have an inactive gene from mother (imprinted).

52. Angelman Syndrome:
- neurological disorder
- seizures
- smiling and happy
- microcephaly
Caused by deletion of a single gene (UBE3A) in 15q11-13 region in mother. This gene is methylated in sperm, and should be ON in eggs. Deletion in the mother results in embryos that lack one gene (from mother) and have an inactive gene from father (imprinted).

53. Heterodisomy = both homologs given from one parent
These conditions can also occur when a zygote receives both chromosomes from one parent (we’ll see how this happens later).
Heterodisomy = both homologs given from one parent
Isodisomy = replicates of one homolog given from one parent
MOTHER
FATHER
Maternal deletion
Paternal Imprint (‘off’)
Isodisomy
Isodisomy
Heterodisomy
Angelman
Angelman
Angelman
Angelman
normal
normal

54. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
3. THE GENETIC ENVIRONMENT - Epigenetics
1. Position Effects
2. Imprinting
3. Maternal Effects
– Genotype of mother determines phenotype of offspring
- by contributing her proteins to the egg, which influence early development of embryo (regardless of the embryo’s genotype for this trait).

55. 3. Maternal Effects:
In Limnaea snails, left-handed(sinistral) coiling is recessive (dd) to dextral (D-).
- But reciprocal crosses yield different results; F1 het’s that had sinistral mothers are sinistral, even though they have the Dd genotype, themselves.

56. 3. Maternal Effects:
In Limnaea snails, left-handed(sinistral) coiling is recessive (dd) to dextral (D-).
- When the Dd heterozygotes (sinistral or dextral) are self-crossed (so the “mother’ is genetically dextral (and produces dextral proteins in her eggs even though it wasn’t in her own body cells) – all the offspring (even the ¼ dd) are dextral.
All dextral, even dd

57. 4. The Cellular Environment of the Egg - Maternal Effects:
All dextral, even dd

58. D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
3. THE GENETIC ENVIRONMENT - Epigenetics
1. Position Effects
2. Imprinting
3. Maternal Effects
– Genotype of mother determines phenotype of offspring
- by contributing her proteins to the egg, which influence early development of embryo (regardless of the embryo’s genotype for this trait).
- through extrachromosomal inheritance – the inheritance of genes “outside” the nuclear chromosomes, rganelles like mitochondria and chloroplasts that have DNA also. These organelles are donated only in the EGG, and so are another potential source of a maternal effect.

59. - through extrachromosomal inheritance – the inheritance of genes “outside” the nuclear chromosomes, in organelles like mitochondria and chloroplasts that have DNA also. These organelles are donated only in the EGG, and so are another potential source of a maternal effect.
Problem of Recognizing the effects because of HETEROPLASMY – there are hundreds of organelles in each cell, so seeing the effects of a loss-of-function mutant are difficult, and may happen only in a few cells in a tissue that happen to randomly get more mutant organelles.

60. Myoclonic epilepsy and ragged red fiber disease (MERFF) in humans
- caused by mitochondrial mutations affecting aerobic respiration (decreasing ATP production).
- cells with lots of these mitochondria don’t function well; this is especially debilitating in tissues with high energy demand, like neural and muscle tissue.
- causes lack of muscle control, seizures, deafness, and dementia
- passed maternally; children of afflicted fathers do not inherit mutant mitochondria.
a) Mild proliferation of mutant mitochondria. b) high proliferation; cell full of mutant mitochondria

61. - through extrachromosomal inheritance – the inheritance of genes “outside” the nuclear chromosomes, in organelles like mitochondria and chloroplasts that have DNA also. These organelles are donated only in the EGG, and so are another potential source of a maternal effect.
Problem of Recognizing the effects because of HETEROPLASMY – there are hundreds of organelles in each cell, so seeing the effects of a loss-of-function mutant are difficult, and may happen only in a few cells in a tissue that happen to randomly get more mutant organelles.

62. 3. THE GENETIC ENVIRONMENT - Epigenetics
1. Position Effects
2. Imprinting
3. Maternal Effects
4. Sex-Limited and Sex-Influenced Traits
- the expression of an autosomal trait depends on the sex of the organism, probably as a consequence of the hormonal environment.
Sex-Limited: only expressed on one sex, males have genes for milk production, but don’t produce milk.
Sex-influenced: more frequent in one sex than another pattern baldness – Bb males bald, Bb females not
FEMALES
MALES BB
Bald (mild, late)
Bald Bb
Not bald bb

63. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects
E. The “Value” of an Allele
1. There are obvious cases where genes are bad – lethal alleles
2. But there are also ‘conditional lethals’ that are only lethal under certain conditions – like temperature-sensitive lethals.
3. And for most genes, the relative value of one allele over another is determined by the relative effects of those genes in a particular environment. And these relative effects may be different in different environments.

64. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia
(incomplete dominance, one gene ‘bad’,
two ‘worse’)
SS Ss ss

65. I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia Survivorship in tropical Africa
(incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’)
two ‘worse’)
SS Ss ss
SS Ss ss

66. Malaria is still a primary cause of death in tropical Africa (with AIDS). The malarial parasite can’t complete development in RBC’s with sickle cell hemoglobin… so one SC gene confers a resistance to malaria without the totally debilitating effects of sickle cell.
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia Survivorship in tropical Africa
(incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’)
two ‘worse’)
SS Ss ss
Survival in U. S.
Survival in Tropics

67. As Darwin realized, selection will favor different organisms in different environments, causing populations to become genetically different over time.
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia Survivorship in tropical Africa
(incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’)
two ‘worse’)
SS Ss ss
SS Ss ss