Genetics and Heredity

Pedigree Analysis

• A pedigree chart is a diagram illustrating the occurrence and appearance of phenotypes for a particular gene or organism
• Traits and phenotypes in ancestors can be tracked from one generation to the next
• These charts use symbols to show family history
• They display the presence or absence of a trait based on family relations
• Pedigree analysis is a tool for studying human genetics and inheritance patterns
• Similar to a family tree, it utilizes symbols to represent family history

Pedigree Symbols

• Square = male

• Circle = female

• Diamond = sex unknown

• Square with a 2 below = two males

• Circle with a 2 below = two females

• Shaded Square = affected male

• Shaded Circle = affected female

• Lined Through Square = deceased male

• Lined Through Circle = deceased female

• Square/Circle with angled line = proband male/female

• Square inside of a Square = examined male

• Circle inside of a Circle = examined female

• Square with hashed line = affected by history

• Circle with hashed line = affected by history (female)

• Single dot in circle = carrier

• Side by side squares connected above = fraternal twins

• Side by side circles connected above = fraternal twins

• Side by side squares connected above and diagonal line connecting both = identical twins (male)

• Side by side circles connected above and diagonal line connecting both = identical twins (female)

• Square with an A inside = adopted male

• Circle with an A inside = adopted female

• Diamond with diagonal line across = lived one day

• Diamond that is filled = stillbirth

• Diamond with a wavy line across = miscarriage

• Circle with a P inside = pregnancy

• Square and circle connected by horizontal line = marriage

• Square and circle connected by double horizontal line = consanguineous marriage

• Square and circle connected by horizontal line with diagonal line going through = extramarital mating

• Square and circle connected by horizontal line with two diagonal lines going through = divorce of separation

• Square and circle not connected = no children

• A karyotype is the full set of chromosomes of a species, including their size, number, and shape

Determining Dominant or Recessive Disorders

• If a disorder is dominant, at least one parent exhibits the disorder
• If a disorder is recessive, neither parent has to show the disorder because they can be heterozygous

Modes of Inheritance

Autosomal Recessive

• Appears in both sexes with equal frequency
• Traits tend to skip generations
• Affected offspring have unaffected parents
• If both parents are heterozygous, roughly 1/4 of offspring will be affected
• More frequent among children of consanguineous marriages
• An autosomal recessive disorder example is cystic fibrosis (CF)
• CF is characterized by mucus accumulation in the lungs and digestive tract
• Two CF carriers have a 25% chance their child will have the disease

Autosomal Dominant

• Appears in both sexes with equal frequency
• Both sexes transmit the trait to their offspring
• Does not skip generations
• Affected offspring have an affected parent unless a new mutation occurred
• When one parent is affected (heterozygous) and the other is unaffected, roughly half the offspring will be affected
• Unaffected parents do not transmit the trait
• Even one dominant allele A results in that specific phenotype

X-Linked Dominant

• Both males and females are affected; often, more females than males

• It Does not skip generations

• An Affected son must have affected mothers

• Affected daughters must have an affected mother or father

• Affected fathers will pass the trait to all of their daughters

• If affected mothers are heterozygous, they will pass the trait on to 1/2 of their sons and 1/2 of their daughters

• In the case of an X-linked trait, a father passes either an X or Y chromosome

• Mothers can only pass the X chromosomes

• When an abnormal allele for a gene that occurs on the X chromosome is dominant over the normal allele, the pattern is described as X-linked dominant

• Vitamin D-resistant rickets are an example of X-linked dominant inheritance

• An affected father passes the disease gene to all his daughters, and none of his sons, because he donates only his Y chromosome to his sons

• If it is the mother, all of her children will have a 50% chance of inheriting the disorder

• If A female is affected, the inheritance pattern would be identical to that of an autosomal dominant inheritance pattern, in which one parent is heterozygous and the other is homozygous for the normal gene

• Traits can vary depending on whether the father or mother is affected

X-Linked Recessive

• More males than females are affected
• Affected sons are usually born to unaffected mothers, and it skips generations
• Approximately 1/2 of carrier mothers' sons are affected
• The trait is never passed from father to son
• All daughters of affected fathers are carriers
• X-linked recessive inheritance is more common because females can be carriers and still have a normal phenotype
• Transmitted diseases include color blindness, hemophilia, and some muscular dystrophy forms
• When the mothers are unaffected carriers with fathers who are normal, none of the daughters would have the disease, but they have a 50% chance of receiving the disease and becoming a carrier from their mother
• With recessive diseases linked to X, males either have the disease, or are genotypically normal-cannot be carriers
• Females, can either be genotypically normal, a phenotypically normal carrier, or affected with the disease
• A daughter can inherit the gene for an X-linked recessive illness if her mother & father is affected by the disease, or her mother is just a carrier
• Disorders commonly affect more males than females, such as color blindness, where 1 in 20 males are affected and only 1 in 400 females

Y-Linked Trait

• Only males will be affected
• It is passed from fathers to all sons
• Does not skip generations
• Dominance is irrelevant because there is only 1 copy

Mitochondrial

• Traits are inherited from mother only
• All children are at risk to be affected or carriers
• Mothers pass mitochondrial genes to all offspring; fathers do not transmit mitochondrial genes
• Sperm very rarely contribute mitochondria to fertilized ova; if mitochondria from a male do enter, they are destroyed

Solving Pedigree Analysis

• First: Look for Mitochondrial Inheritance
  • Females transmit the disease to all of their offspring (both males and females)
  • If The male has one, he does not transmit the disease and only the females transmit it

Solving for recessive and dominant

• Second: Look if the gene is Dominant, Recessive
  • At least one member in all generations will have the disease in dominant
  • The Affecting parents can produce offspring
• Third: Look if The gene is X-linked or Autosomal or Y-linked
  • Y-linked occurs only with men

Terminology

• Allele: Alternative forms of a gene occupying a specific locus
• Autosomal chromosome: In humans, the 22 pairs of non-sex chromosomes
• Autosomal dominant: Pattern of inheritance corresponds to a gene on an autosomal chromosome
• Autosomal recessive: Pattern of recessive inheritance that corresponds to a gene on an autosomal chromosome
• Carrier: Heterozygous individual without symptoms of a recessive disorder but can transmit it to offspring
• Codominance: is a pattern of inheritance corresponding to the simultaneous expression of two alleles
• Dominant: A trait expressed in both homozygous and heterozygous forms
• Dominant lethal: Inheritance pattern where an allele means certain death
• Genotype: The complete genetic makeup of an individual
• Heterozygous: Having two different alleles for a gene
• Homozygous: Having two identical alleles for a given gene

Explore patterns of inheritance including autosomal recessive, autosomal dominant, and X-linked dominant inheritance. Study the probability of inheriting genetic conditions and how to interpret pedigree charts. Understand the role of chromosomes in genetic transmission and the information provided by karyotypes.