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Transcript
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: – BA, which is red ash and is the most dominant – B, which is blue and is recessive to BA but dominant to b – b, which is chocolate and recessive to both BA 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: XR or Xr • 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 Genotype Phenotype example of AIA or IAi I A multiple alleles in humans IBIB or IBi B • The following genotypes and I AI B AB phenotypes can occur: ii O 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 2nd 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