Biology Chapter 12 Patterns of Heredity & Human Genetics

12.1 Pedigrees Pedigree- a graphic representation of genetic inheritance –There are some common symbols used in pedigrees as shown Known heterozygotes (carrier)

12.1 Analyzing a Pedigree From a pedigree, you can determine: –whether the trait is dominant or recessive –The phenotypes of individuals –the genotypes of homozygous recessive individuals You can only predict genotypes of those that exhibit the dominant phenotype based on the phenotypes of their offspring and parents

12.1 Recessive Heredity Most genetic disorders result from a recessive allele combination Examples: –Cystic fibrosis: common among white Americans, results in the formation of thick mucus in the lungs and digestive tract –Tay-Sachs: results in the absence of an enzyme that breaks down fats, so it gets stored in the cells –Phenylketonuria (PKU): results in the absence of an enzyme that converts phenylalanine to tyrosine and damages the central nervous system

12.1 Dominant heredity Some traits are inherited because of having 1 or 2 dominant alleles –Examples include: cleft chin, widows peak, earlobe attachment, eye shape, etc. –Genetic disorders can also be dominant, such as Huntington’s disease, where certain areas of the brain (CNS) break down Huntington’s does not appear until the 30’s and 40’s so it is often passed on to offspring before the individual is aware that they have it

12.2 Complex Patterns of Inheritance Mendel’s studies on heredity use only simple inheritance-having dominant and recessive traits There are other inheritance patterns: –Incomplete dominance –Co-dominance –Multiple alleles –Sex linked inheritance –Polygenic inheritance

12.2 Incomplete Dominance In incomplete dominance, the phenotype of the offspring is intermediate between that of the parents –For example: a red snapdragon crossed with a white snapdragon results in pink snapdragons –The alleles for incomplete dominance are both capital, with one having a prime symbol For example R (red), R‘ (white) and RR’ (pink)

12.2 Co-dominance In co-dominance, the phenotype of the offspring includes both of the parental phenotypes –For example: a black rooster crossed with a white hen results in black and white offspring, called checkered –The alleles for co-dominance are two different capital letters For example: B (black) and W (white) and BW (checkered)

12.2Multiple Alleles Multiple alleles are traits controlled by more than 2 alleles –Each organisms will only have 2 of these alleles, but there are more than 2 possibilities For example pigeons can be: –B A, which is red ash and is the most dominant –B, which is blue and is recessive to B A but dominant to b –b, which is chocolate and recessive to both B A and B

12.2 Sex Determination The gender of offspring is determined by the sex chromosomes –In humans there should be 2 sex chromosomes XX results in a female XY results in a male –Because only the male can contribute different alleles, the father’s sex chromosome determines the sex of the baby The other chromosomes are identical and called autosomes –Humans have 22 pairs (or 44) of these chromosomes

12.2 Sex Linked Inheritance Some traits are controlled by genes located on sex chromosomes –The alleles for sex linked traits are written as superscripts on the sex chromosomes For example: X R or X r X linked traits can be passed on to male or female offspring Y linked traits can only be passed on from father to son

12.2 Polygenic Inheritance Polygenic inheritance occurs when multiple genes (with 2 alleles each) can control for a trait –This is present when traits can occur over a wide range, such as height and skin color –These genes can be on the same or different chromosomes

12.2 Environmental Influence The environment of an organism (both internal and external) can also determine the phenotype that is expressed –Examples of external factors include light, temperature, infectious agents and nutrition For example the artic fox has brown fur when it is warm and white fur when it is cold to help them blend in with their surroundings –Examples of internal agents include hormones associated with gender For example, mountain sheep males have bigger horns than mountain sheep females

12.3 Co-dominance in Humans Sickle Cell disease is an example of co-dominance in humans –The sickle shaped cells result from a change in the protein hemoglobin Normal red blood cells are disc shaped Homozygous sickle cell individuals have red blood cells that are all sickle shaped (half-moon shaped) Heterozygous individuals have some disc shaped and some sickle shaped red blood cells

12.3 Multiple Alleles in Humans Blood Type is an example of multiple alleles in humans The following genotypes and phenotypes can occur: GenotypePhenotype I A I A or I A iA I B I B or I B iB IAIBIAIB AB iiO

12.3 Sex Linked Traits in Humans Sex linked traits can be inherited on the X chromosome or the Y chromosome –If a male receives a recessive allele on the X chromosome, the trait will be expressed –A female must receive a recessive allele on both X chromosomes to express the trait –If the recessive allele is on the Y chromosome, it can only be passed from father to son Females cannot exhibit sex linked traits on the Y chromosome

12.3 X-linked human traits Red-Green colorblindness –Recessive –Individuals cannot tell the difference between red and green Hemophilia –Recessive –Individuals do not have enough clotting factor in their blood, so it does not clot

12.3 Polygenic Traits in Humans Skin Color –Intermediate skin colors exist in the first generation, with the original skin colors as well as the intermediates appearing in the 2 nd generation Eye Color –Eyes can be a range of colors, including solids and mixtures of colors

12.3 Changes in Chromosome Numbers Karyotypes are diagrams that show the number and types of chromosomes –Individuals can have an extra autosome, that results in diseases such as in Down’s syndrome –Individuals can have an extra sex chromosome that results in diseases such as Kleinfelter’s