Mendel’s law and exceptions to the second law
George Mendel
Inheritance and peas
The passing of traits
Dominant and recessive alleles (the alternative forms of a gene are alleles)
Each trait depends on factors -> alleles form father and mother
Combination of alleles: genotype
Result of genotype: phenotype
Homozygous: identical alleles at locus
Heterozygous: different alleles at locus
Punnett Square diagram
Used to predict genotypes of a cross or breeding experiment
Useful to determine the probability of an offspring having a particular genotype
Dom: yellow allele
Rec: green allele
Darwin: blending inheritance
Versus Mendels first law
Darwin: theory of pangenesis: changes
in living conditions alarmed replicas of
the gemmules (parental cells) in
animals’ reproductive systems so their
offspring were more variable than they
would have been
Mendel concluded
There are two elements or heredity for each trait in each individual
These two elements segregate (separate) during reproduction
Offspring receives one of the two elements from each parent
In statistics:
,Mendel’s laws
1. Law of dominance: rather than both alleles contributing to a phenotype, the dominant
allele will be expressed exclusively
2. The law of segregation: offspring receives one of the two elements of a trait from each
parent, either dominant or recessive (a recessive element is only expressed if both alleles
are recessive)
3. The law of independent assortment: the inheritance pattern of one trait will not affect
the inheritance pattern of another trait
Mendelian traits/ disorders
Single-gene mutations changing a person
- Huntington’s disease
- PKU: Phenylketonuria
- Scurvy
Huntington’s disease (DOM)
Brain disorder causing progressive deterioration of brain cells that leads to severe
incapacitation and eventual death
Symptoms: uncontrollable movements, abnormal balance, slurred speech, thinking
difficulties, personality change
No cure or effective treatment available
- Huntington’s is due to a single gene with the allele for HD dominant -> if one of the
parents has one HD dominant (so all other three alleles are hd), 50% of the offspring
is at risk of Huntington’s disease
Phenylketonuria (REC)
Metabolic disorder that prevents normal metabolization in phenylalanine (Phe) which is
present in protein rich foods
Damage occurs to the nervous system and leads to mental retardation
Prevention can be accomplished by dieting
- Phenylketonuria is inherited as a single gene, the allele that causes pku is recessive.
When both parents are carriers, the risk of pku for their children is 25%
Scurvy
Caused by a single gene in humans, but not considered a genetic disorder in humans
because everyone has that defective gene!
Treated by diet, bus does not make sense to care about Mendel’s law for this disease
, Exceptions to Mendel’s second law
The law holds for: The law doesn’t hold for:
-gens on different chromosomes -genes on close proximity on the
-genes that are far apart on the same chromosome same chromosome
If any two traits have genes in close proximity, then the traits will not be passed down
independently and there will be a bias in the Punnett square probabilities
Linkage: when certain genes are inherited together and thus localized on the same
chromosome
Recombination: chromosomes exchange parts during meiosis
Meiosis: chromosomes duplicate to form sister chromatids
Centimorgan: distance between two loci on which recombination depends
X-linked recessive inheritance = refers to genetic conditions associated with mutations in
genes on the X chromosome. A male carrying such a mutation will be affected, because he
carries only one X chromosome. A female carrying a mutation in one gene, with a normal
gene on the other X chromosome, is generally unaffected.
- Eg colorblindness (skip a generation phenomenon)
X-linked dominant inheritance = refers to genetic conditions associated with mutations in
genes on the X chromosome. A single copy of the mutation is enough to cause the disease in
both males (who have one X chromosome) and females (who have two X chromosomes)
Inheritance of quantitative traits involves two or more nonallelic genes (multiple genes or
polygenes); the combined action of these genes, as in fenced by the environment, produces
the phenotype. The effect of individual genes on the trait is not apparent.
Estimate of morbidity risk: change of getting a disease over a lifetime
Polygenic traits: influenced by multiple genes (Mendel’s laws also apply here)
Vulnerability threshold model: assumes that dichotomous disorder (disorder or not) are due
to underlying genetic vulnerabilities which are normally distributed, and that the disorder
only occurs when a vulnerability threshold is crossed
George Mendel
Inheritance and peas
The passing of traits
Dominant and recessive alleles (the alternative forms of a gene are alleles)
Each trait depends on factors -> alleles form father and mother
Combination of alleles: genotype
Result of genotype: phenotype
Homozygous: identical alleles at locus
Heterozygous: different alleles at locus
Punnett Square diagram
Used to predict genotypes of a cross or breeding experiment
Useful to determine the probability of an offspring having a particular genotype
Dom: yellow allele
Rec: green allele
Darwin: blending inheritance
Versus Mendels first law
Darwin: theory of pangenesis: changes
in living conditions alarmed replicas of
the gemmules (parental cells) in
animals’ reproductive systems so their
offspring were more variable than they
would have been
Mendel concluded
There are two elements or heredity for each trait in each individual
These two elements segregate (separate) during reproduction
Offspring receives one of the two elements from each parent
In statistics:
,Mendel’s laws
1. Law of dominance: rather than both alleles contributing to a phenotype, the dominant
allele will be expressed exclusively
2. The law of segregation: offspring receives one of the two elements of a trait from each
parent, either dominant or recessive (a recessive element is only expressed if both alleles
are recessive)
3. The law of independent assortment: the inheritance pattern of one trait will not affect
the inheritance pattern of another trait
Mendelian traits/ disorders
Single-gene mutations changing a person
- Huntington’s disease
- PKU: Phenylketonuria
- Scurvy
Huntington’s disease (DOM)
Brain disorder causing progressive deterioration of brain cells that leads to severe
incapacitation and eventual death
Symptoms: uncontrollable movements, abnormal balance, slurred speech, thinking
difficulties, personality change
No cure or effective treatment available
- Huntington’s is due to a single gene with the allele for HD dominant -> if one of the
parents has one HD dominant (so all other three alleles are hd), 50% of the offspring
is at risk of Huntington’s disease
Phenylketonuria (REC)
Metabolic disorder that prevents normal metabolization in phenylalanine (Phe) which is
present in protein rich foods
Damage occurs to the nervous system and leads to mental retardation
Prevention can be accomplished by dieting
- Phenylketonuria is inherited as a single gene, the allele that causes pku is recessive.
When both parents are carriers, the risk of pku for their children is 25%
Scurvy
Caused by a single gene in humans, but not considered a genetic disorder in humans
because everyone has that defective gene!
Treated by diet, bus does not make sense to care about Mendel’s law for this disease
, Exceptions to Mendel’s second law
The law holds for: The law doesn’t hold for:
-gens on different chromosomes -genes on close proximity on the
-genes that are far apart on the same chromosome same chromosome
If any two traits have genes in close proximity, then the traits will not be passed down
independently and there will be a bias in the Punnett square probabilities
Linkage: when certain genes are inherited together and thus localized on the same
chromosome
Recombination: chromosomes exchange parts during meiosis
Meiosis: chromosomes duplicate to form sister chromatids
Centimorgan: distance between two loci on which recombination depends
X-linked recessive inheritance = refers to genetic conditions associated with mutations in
genes on the X chromosome. A male carrying such a mutation will be affected, because he
carries only one X chromosome. A female carrying a mutation in one gene, with a normal
gene on the other X chromosome, is generally unaffected.
- Eg colorblindness (skip a generation phenomenon)
X-linked dominant inheritance = refers to genetic conditions associated with mutations in
genes on the X chromosome. A single copy of the mutation is enough to cause the disease in
both males (who have one X chromosome) and females (who have two X chromosomes)
Inheritance of quantitative traits involves two or more nonallelic genes (multiple genes or
polygenes); the combined action of these genes, as in fenced by the environment, produces
the phenotype. The effect of individual genes on the trait is not apparent.
Estimate of morbidity risk: change of getting a disease over a lifetime
Polygenic traits: influenced by multiple genes (Mendel’s laws also apply here)
Vulnerability threshold model: assumes that dichotomous disorder (disorder or not) are due
to underlying genetic vulnerabilities which are normally distributed, and that the disorder
only occurs when a vulnerability threshold is crossed
