Ch. 11 - Complex Inheritance & Human Heredity

Section 2: Complex Patterns of Inheritance It makes absolutely no sense whatsoever to continue if we still don't know what the word "allele" means. Allele = a form of a gene which codes for one possible outcome of a phenotype. Example: In Mendel's pea investigations, he found that there was a gene that determined the color of the pea pod. One form of it (one allele) creates yellow pods, & the other form (allele) creates green pods. Two possible phenotypes of one trait (pod color) are determined by two alleles (forms) of the one "color" gene.

The what’s the difference between a GENE, a TRAIT, and an ALLELE The what’s the difference between a GENE, a TRAIT, and an ALLELE?! Aren’t they all the same thing? The answer is Yes and No. A gene is a stretch of DNA or RNA that determines or codes for a certain characteristic. A trait is the characteristic the gene is coding for, or in simpler terms, the phenotype. When homologous chromosomes pair up during meiosis, they match up based on their gene’s location. Because there are two chromosomes, you result in two (or more) alternate forms for each gene; an allele is one of these forms of a gene.

The what’s the difference between a GENE, a TRAIT, and an ALLELE The what’s the difference between a GENE, a TRAIT, and an ALLELE?! Aren’t they all the same thing? Example: The gene for eye color is located near the tip of the 11th chromosome. A blue-eye man’s gene for eye color is ‘b’. A brown-eyed female’s gene for eye color is ‘B’. The letter (B) is the gene, the letters ‘B’ and ‘b’ are different alleles for eye color. The males blue eyes are a trait and the female’s brown eyes are a trait, or their phenotypes.

Inheritance of some traits does not always follow Mendel’s laws! Geneticists state there are 6 different types of complex inheritance patterns that go against Mendelian’s genetics: Incomplete Dominance Codominance Multiple Alleles Epistasis Sex-linked Traits Polygenic Traits

Incomplete Dominance If two alleles exhibit incomplete dominance, the heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes. With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

Incomplete Dominance Examples: Flower color in snapdragons can be red, white, or pink. Hair texture in humans - straight-haired man and a curly-haired female produce a baby wavy hair.

Codominance If two alleles exhibit codominance, both alleles are expressed (or show) in the heterozygous condition. With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. The meaning of the prefix "co-" is "together". Cooperate = work together Coexist = exist together Cohabitat = habitat together

Codominance Examples: Fur color in cows can be white, brown, or white and brown spotted. Sickle-cell disease in humans produces normal, healthy red blood cells AND sickle-shaped red blood cells.

Multiple Alleles A form of inheritance that is determined by more than two alleles which results in four or more phenotypes for one gene is referred to as multiple alleles. Example: Blood groups in humans - There are 3 alleles for blood type: IA, IB, and i. There are 4 phenotypes for blood type. A person can be Type A, Type B, Type AB, or Type O.

Multiple Alleles Example: Coat color in rabbits - There are 4 alleles for coat color in rabbits that results in 4 different phenotypes: Full color, Chinchilla, Himalayan, & Albino.

Epistasis A form of inheritance in which there are 2 sets of alleles that cause observable variation in a trait because one allele masks the effects of another allele. Example: Coat color in Labrador Retrievers – A lab’s coat color is determine by two sets of alleles: ‘E’ codes for whether pigment is present or not, and ‘B’ codes for the darkness of that pigment. Both alleles code for the same gene (coat color). EEBB will give you a black lab, EEbb will produce a chocolate lab, and eebb will give you a yellow lab.

Sex-linked Traits Traits controlled by the genes located on the X chromosome ONLY! **Humans can use a pedigree to track these traits throughout generations and predict offspring likelihood to inherit trait.

Let’s refresh your brain! In humans, each somatic cell (body cell) contains 46 chromosomes. In humans, each gamete contains 23 chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes). Males contain the sex chromosomes XY; females contain the sex chromosomes XX. Because males only have one X chromosome, they are affected by recessive X-linked traits more often than females. Because females have two X chromosomes, they are less likely to express an X-linked trait because the other X chromosome may mask the recessive trait.

Sex-linked Traits Example: What you see: What they see: Example: Red-green color blindness in humans - The trait for red-green color blindness is a recessive X- linked trait. Because it is recessive, males are far more likely to have this type of color blindness. It is very rare in females -- the parents of a female with red-green color blindness must BOTH be red- green color blind.

Sex-linked Traits Example: Hemophilia - Recessive X-linked disorder that results in the delayed inability to clot one’s own blood; before modern medicine, people who had Hemophilia would bleed out from a simply falling down and cutting their skin open. Also called the “Royal Disease” after studying the pedigree of Queen Victoria’s family.

Polygenic Traits Traits that result from the interaction of multiple pairs of genes. Examples: Skin color - in certain genes, the more dominant alleles present results in darker skin color; the more recessive alleles results in paler skin color. Height Fingerprint pattern

Let’s do some practice!

Section 1: Basic Patterns of Human Inheritance Tracing human inheritance can show how a trait was passed down from one generation to the next.

Recessive Genetic Disorders Recall that a recessive trait is expressed when the individual is homozygous recessive for that trait (therefore, those with at least one dominant allele will not express it). An individual who is heterozygous for a recessive disorder is called a carrier.

Recessive Genetic Disorders Examples of Recessive Genetic Disorders: Cystic Fibrosis, Tay-Sachs Disease, Galactosemia (lactose-intolerance), and Albinism.

Recessive Genetic Disorders What is/are the genotype(s) of people with a recessive genetic disorder? Use the letter ‘A’. Answer = aa What is/are the genotype(s) of people with a WITHOUT a recessive genetics disorder? Use the letter ‘A’. Answer = AA or Aa ***If two people who are both heterozygous for any of the above traits married… (Draw a Punnett Square) What is the chance their child will be born with the disorder? Without? What is the chance their child will be a carrier for the disorder?

Dominant Genetic Disorders A dominant trait is expressed when the individual is either Homozygous or Heterozygous for that trait (therefore, only those with two recessive alleles will not express/show it). An individual cannot be a recessive for a dominant disorder.

Dominant Genetic Disorders Examples of Dominant Genetic Disorders: Huntington’s disease and Achondroplasia (dwarfism).

Dominant Genetic Disorders What is/are the genotype(s) of people with a dominant genetic disorder? Use the letter ‘B’. Answer = BB and Bb What is the genotype(s) of people WITHOUT a dominant genetic disorder? Answer = bb ***If a man who is heterozygous for any of the above traits and a normal woman in got married… (Draw a Punnett Square) What is the chance their child will be born with the disorder? What is the chance their child will be born WITHOUT with the disorder?

Pedigrees Scientists called genealogists study a human’s family history using a pedigree, a diagram that traces a particular trait through several generations. Symbols are used to illustrate the inheritance of the trait. Males are represented by squares; females by circles. One who expresses the trait being studied is represented by a dark (filled in) square or circle.

Pedigrees Pedigrees use a special “tree-like” format to show heredity throughout several generations. A horizontal line between two symbols shows that these individuals are the parents of the offspring listed below them. Offspring are listed in descending birth order from left to right and are connected to each other and their parents with lines. A numbering system is used: Roman Numerals represent generations, and individuals are numbered by birth order using Arabic Numbers.

Analyzing Pedigrees – Recessive Disorders A circle or square that is NOT darkened in indicates a person that has the genotype Homozygous Dominant. A darkened in circle or square indicates a person that has the genotype Homozygous Recessive. If a person has the genotype Aa, the circle or square is half-filled. Inferring Genotypes for Recessive Genetic Disorders If a person has the recessive trait, each parent MUST have at least one recessive allele.

Analyzing Pedigrees – Dominant Disorders A circle or square that is NOT darkened in indicates a person that has the genotype Homozygous Recessive. The genotype of a person who is represented by a darkened in circle or square is Homozygous Dominant. A person with the genotype Aa is NOT represented by a half-darkened in circle or square – because it is NOT possible to be a carrier for dominant genetic disorders.

“Royal Disease” - Pedigree

Section 3: Chromosomes & Human Heredity How we study our chromosomes and what they can tell us.

Karyotyping Karyotype: a micrograph in which the pairs of homologous chromosomes are arranged in decreasing size. Scientists use karyotypes to study homologous chromosomes by taking a picture of them during metaphase.

Karyotyping A karyotype is often done to determine if the offspring has the correct number of chromosomes. An incorrect number of chromosomes indicates that a child will have a genetic condition, like Down Syndrome.

Notice: A person should have 22 pairs of homologous chromosomes (autosome) & 1 pair of sex chromosomes (XX or XY) = 23 pairs.

Notice: A person with Down Syndrome has an extra chromosome on #21 Notice: A person with Down Syndrome has an extra chromosome  on #21. Instead of a pair, this person has 3 chromosomes. This is referred to a condition called Trisomy.  (tri = three) Trisomy results from Nondisjunction: when sister chromosomes fail to separate during meiosis, producing sex cells with extra chromosomes.

Nondisjunction Because their cells do not divide as efficiently as they used to, older males and females are more likely to produce egg and sperms cells that have undergone nondisjunction.