Genetics: Genes and Chromosomes

Genetics Overview

• Gregor Mendel is considered the "Father of Genetics".
• The principle of independent assortment states that the inheritance of one trait does not affect the inheritance of another.
• Genetics is the study of how traits are passed from parent to offspring.
• Traits are determined by genes located on chromosomes; a gene is a segment of DNA that determines a specific trait.

Chromosomes and Genes

• Chromosomes come in homologous pairs, meaning genes also come in pairs.
• Homologous pairs contain matching genes, with one gene from the female parent and one from the male parent.
• Humans have 46 chromosomes, or 23 pairs: one set of 23 from the sperm (dad) and one set of 23 from the egg (mom).
• Alleles are different versions or possibilities of genes for the same trait, like blue or brown eyes.

Dominant and Recessive Genes

• A dominant gene prevents the expression of another gene, and is symbolized by an uppercase letter.
• A recessive gene is not expressed when a dominant gene is present, and is symbolized by a lowercase letter.
• Individuals must have two copies of the recessive allele for the recessive trait to be expressed

Genotype and Phenotype

• Homozygous individuals have two identical alleles for a trait, and can be either homozygous dominant (TT) or homozygous recessive (tt).
• Heterozygous individuals have one dominant and one recessive allele (Tt), also known as a hybrid.
• The genotype is the actual gene makeup of an organism (e.g., TT, Tt, tt).
• The phenotype is the physical appearance resulting from the gene makeup (e.g., hitchhiker's thumb or straight thumb).

Punnett Squares and Ratios

• Punnett Squares are useful to predict the potential genetic combinations of offspring.
• In a cross between a heterozygous male (Bb) and a homozygous recessive female (bb), the possible offspring genotypes are Bb and bb, with a genotypic ratio of 2:2 and a phenotypic ratio of 2 black: 2 white.
• When crossing two hybrid mice (Bb x Bb) the genotypic ratio is 1 BB : 2 Bb : 1 bb, and the phenotypic ratio is 3 black : 1 white.

Dihybrid Crosses

• Dihybrid crosses involve the inheritance of two traits.
• An example involves rabbits with black coats (B) dominant over brown (b) and straight hair (H) dominant over curly (h).
• Crossing two hybrid rabbits (BbHh x BbHh) produces a phenotypic ratio of 9:3:3:1 for the offspring.

Sex Determination

• Humans have 46 chromosomes or 23 pairs.
• 22 pairs of chromosomes are homologous and called autosomes determining body traits.
• 1 pair of sex chromosomes determines sex.
• Females have homologous sex chromosomes labeled XX.
• Males have different sex chromosomes labeled XY.
• The probability of having a female baby is 50%, and the male determines the sex of the child.

Non-Mendelian Genetics

• Incomplete dominance occurs when one allele is not completely dominant over another, leading to a blending of traits.
• In carnations, where red (R) is incompletely dominant over white (W), a hybrid (RW) will be pink.
• The result of crossing 2 pink RW flowers gives a genotypic ratio of 1 RR : 2 RW : 1 WW
• The phenotypic ratio is 1 red : 2 pink : 1 white.
• Codominance is when both alleles are expressed.
• In certain chickens, black and white feathers are codominant, resulting in heterozygous chickens having black and white speckled feathers.

Sex-Linked Traits

• Sex-linked genes are located on the X chromosome.
• They show up more often in males because males have only one X chromosome.
• Colorblindness is an inability to distinguish between certain colors.
• Colorblindness is a recessive sex-linked disorder.
• Hemophilia is a condition the blood does not clot normally.
• If a female carrier for colorblindness marries a male with normal vision, phenotypes will be 2 normal vision females, 1 normal vision male and 1 colorblind male.

Pedigrees

• Pedigrees are graphic representations of how a trait is passed from parents to offspring.
• Circles represent females and squares represent males in pedigrees
• Horizontal lines between a circle and square indicate marriage.
• Vertical lines and brackets connect parents to offspring.
• Shaded shapes indicate a person has the trait, and unshaded shapes indicate a person does not have the trait.
• Partial shading indicates a carrier who is heterozygous.

Complex Inheritance

• Some genes have 3 or more alleles for a single trait
• In humans, blood type is determined by 3 alleles: A, B, and O
• Every person can only inherit 2 alleles
• Alleles A and B are dominant (codominance)
• Allele type O is always recessive
• Blood type A = AA or AO
• Blood type B = BB or BO
• Blood type AB = AB
• Blood type O = OO
• Children from a female of blood type AB, and a male of blood type O blood will have offspring that are A or B blood type only

Mutations

• A mutation is a sudden genetic change in the base pair sequence of DNA.
• Harmful mutations lessen an organism ability to survive and can cause genetic disorders, cancer, and death.
• Beneficial mutations allows an organism to better survive and provides genetic variation
• Neutral mutations are neither harmful or helpful.
• Mutations can occur in two ways; chromosomal mutation, or gene/point mutation.

Chromosomal Mutations

• Are less common than a gene mutation, but are more drastic.
• Affect the entire chromosome.
• Impacts many genes
• Happens from failure of the homologous chromosomes to separate during meiosis
• Chromosome pairs are no longer the same as they loose or gain genetic code - altering their shape

Chromosomal Abnormality Examples

• Down's syndrome (Trisomy 21) results in 47 chromosomes, with an extra chromosome at pair #21.
• Turner's syndrome results in only 45 chromosomes, with a missing sex chromosome (X).
• Girls suffer short stature, slow growth, and heart problems.
• Klinefelter's syndrome results in 47 chromosomes, with extra X chromosomes (XXY).
• Boys suffer low testosterone levels, underdeveloped muscles, and sparse facial hair.
• Having an extra set of chromosomes is fatal in animals, but causes plants to be larger and hardier.

Gene or Point Mutation

• Are more common and less drastic
• Only one gene is altered

Gene Mutation Examples

• Recessive gene mutations; Sickle cell anemia red blood cells are sickle shaped. Instead they are round, cannot carry enough oxygen
• Heterozygous people are protected from malaria
• Cystic fibrosis is caused by mutated genes that builds up mucus in the lungs
• Tay-Sachs Disease is deterioration of the nervous system and early death
• Phenylketonuria is a build up of amino acids in milk due to it not being properly digested. This is tested for during birth to avoid mental retardation
• Dominant gene mutations are Huntington's disease that is the gradual loss of brain tissue which results in death during middle age, and Dwarfism which is variety of skeletal abnormalities

Detecting Genetic Disorders

• A karyotype is a picture of an individual's chromosomes.
• Amniocentesis is a procedure that involves removing amniotic fluid surrounding the embryo for analysis.