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Dominant & Recessive Traits Chapter 12 DOMINANT TRAITS eye coloring brown eyes RECESSIVE TRAITS grey, green, hazel, blue eyes vision farsightedness normal vision normal vision normal vision normal vision nearsightedness, night blindness, color blindness* hair dark hair non-red hair curly hair full head of hair widow's peak blonde, light, red hair red hair straight hair baldness* normal hairline facial features dimples unattached earlobes freckles broad lips no dimples attached earlobes no freckles thin lips Dominant Trait Recessive Trait Appendages extra digits fused digits short digits fingers lack 1 joint limb dwarfing clubbed thumb double-jointedness bent pinky normal number normal digits normal digits normal joints normal proportion normal thumb normal joints straight pinky other immunity to poison ivy normal pigmented skin normal blood clotting normal hearing normal hearing & speaking normal- no PKU *sex-linked traits susceptibility to poison ivy albinism hemophilia* congenital deafness deaf mutism phenylketonuria (PKU) Longer 2nd toe is dominant over 2nd toe shorter than big toe. Mendel’s experiment Gregor Mendel conducted experiments that used monohybrid crosses; carried out in 3 steps Monohybrid cross- studies 1 trait Step 1: Mendel allowed pea plants to selfpollinate for several generations to get offspring that are true-breeding (purebred). He used true-breeding plants as the first generation in his experiment. These were called the parental (P) generation Mendel’s experiment Step 2: Mendel crossed 2 P generation plants with contrasting traits. This produced offspring called the first filial generation (F1 generation). Mendel recorded the # of F1 generation plants with each trait. Step 3: Mendel allowed the F1 generation to self-pollinate, producing a second filial (F2) generation. He then recorded the # of F2 generation plants with each trait. Mendel’s Results F1 generation- 100% displayed the same trait for a given character (the dominant trait) F2 generation- the trait that disappeared in the F1 generation reappeared. The ratio of the characters in this generation were approximately 3:1 (dominant:recessive). Notice this # is approximate. Why? Law of Segregation When an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles. Patterns in Inheritance? Mendel conducted another experiment to see if traits were passed down in a pattern. He did this by conducting a dihybrid cross. dihybrid cross- involves 2 characters He found that the inheritance of one trait did not affect the inheritance of any other trait. This is because of the Law of Independent Assortment. Law of Independent Assortment During gamete formation (meiosis) the alleles of each gene segregate independently. Alleles can “mix and match” i.e. Round seeds can be green or yellow (as can wrinkled seeds) Exceptions: Genes that are located close together on the same chromosome will rarely separate independently. These genes are linked genes. Sex Linked Traits •Genes located on one of the sex chromosomes (X or Y) •Most sex linked traits are on the X chromosome because it is much longer than the Y chromosome •More common in males since they only have one X chromosome Examples: colorblindness, hemophelia More complicated traits Most traits do not follow Mendel’s pattern of inheritance because most genes either have more than 2 possible alleles OR are controlled by more than 1 gene. Examples: Polygenic inheritance Incomplete dominance Multiple alleles Codominance Polygenic Inheritance Characters that are controlled by more than one gene are polygenic traits. Examples: eye color (amount of greenness or browness of the eye), height, skin color Incomplete dominance 2 dominant alleles occur. Neither allele is completely dominant over the other. Example: snapdragons Multiple Alleles Genes that have 3+ possible alleles have multiple alleles. Only 2 alleles for a gene can be present in one individual. The determination of dominance in these cases can be very complex. Example: blood type (alleles= IA, IB, i) Codominance 2 traits can appear at the same time for some characters, leading to codominance. In these cases, both alleles for the same gene are fully expressed. Examples: applies to blood type AB, roan fur color in cattle Pedigree A family history that shows how a trait is inherited over several generations. A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. A pedigree can help us learn about sexlinked traits, dominance, & heterozygosity. Affects of the environment Some phenotypes can be affected by conditions in the environment (such as nutrients & temperature). Examples: color of the arctic fox is affected by temperature, height in humans can be affected by nutrition, exposure to too much oxygen in premature babies leads to blindness while too little oxygen leads to brain damage, drugs that cause birth defects (thalidomide prescribed to treat morning sickness in pregnant women), sunlight & butterfly wing color