Genetics Chapter 11

Basic Terms Trait – an inheritable physical characteristic May be internal or external Ex: Eye color, hair color, blood type, personality Gene - are chemical factors (Segments of DNA) that code for physical traits Allele - Different versions of the genes that may be inherited 2 alleles are inherited for each trait; one from mom and one from dad Some alleles are Dominant and some are Recessive Dominant alleles are represented by a CAPITAL LETTER Recessive alleles are represented by a lowercase letter Ex: Brown hair color is dominant over blonde hair color; if one of each allele is inherited, only the brown hair color will be seen Phenotype – The physical appearance of the trait Genotype – The genetic description of the trait Homozygous Dominant – 2 dominant alleles were inherited Homozygous Recessive – 2 recessive alleles were inherited Heterozygous – 1 dominant and 1 recessive allele were inherited

Gregor Mendel Austrian monk who did his work in a monastery during the mid-1800’s Fertilization is the process of male and female reproductive cells joining to form a new organism Mendel became interested in knowing why some pea plants had white flowers (a trait) and some had purple…or, why some pea plants were tall and others short He fertilized his pea plants and examined the traits of the offspring which later became the basis for modern genetics

More Mendel Mendel started his experiments by looking at flower color in pea plants Flower color is an example of a Trait He “self-fertilized” (a form of asexual reproduction) the purple flower plants until ALL of the offspring continuously produced purple flowers. These purple flower plants were True breeding Their genotype was Homozygous Dominant (PP)

Mendel Cont. Next, Mendel self-fertilized plants that had white flowers, until all the offspring produced had white flowers These offspring were true breeding for white flowers The white flower plants were Homozygous recessive (pp) He then used a sample of the true breeding white flowers and the true breeding purple flowers as the “parent generation” in his experiments

The Laws of Genetics Law of Dominance: The dominant allele will always show in the phenotype unless it is not present. In other words, the dominant allele masks the presence of a recessive allele Law of Segregation: Alleles separate during gamete formation (meiosis) This provides each sex cell with one copy of a gene instead of the usual two Law of Independent Assortment: Genes are inherited separately from one another due to “crossing-over” during meiosis and chromosome separation during meiosis

True-Breeding Pea Plants pp = Homozygous Recessive

Cross-Fertilization Mendel took the pollen from a purple flower plant and used it to fertilize a white flower plant. Predict what the offspring plants looked like They were all Purple! Their genotypes were Heterozygous (Pp) These Pp plants were the F1 Generation PP pp All Pp PG

Cross-Fertilization Again Predict what happened next when Mendel crossed two Heterozygous Purple plants (Fertilized one plant with the pollen of the other) 75% of the offspring were Purple, 25% of the offspring were White These offspring were the F2 generation

Types of Dominance Some Dominant alleles are not completely dominant These genes only partially mask the recessive allele if it is present Therefore, these alleles may show Incomplete Dominance Ex: A pink flower results from a cross between a red flower and white flower Some genes may combine 2 dominant alleles These phenotypes show up together at the same time Therefore, these alleles are Codominant Ex: A Black and white cow results when a black bull is mated with a white cow

Examples of Incomplete Dom. And Codom. Incomplete Dominance:Codominance RRrr Rr

Exceptions to the Rule Multiple Alleles: Some genes have multiple alleles EX: Blood type alleles I A I B i Both I A and I B are Dominant alleles whereas “i” is a recessive allele Not only are there 3 possible alleles (instead of the usual 2 alleles for most traits), but I A and I B can show codominance

Blood Type Phenotypes There are six possible genotypes of blood type ii, I A i, I B i, I A I A, I B I B, I A I B These lead to 4 possible phenotypes A, B, AB or O Side notes: Type O is the Universal Donor, this means anyone can receive this blood in transfusion, however, Type O patients can ONLY receive Type O blood Type AB is the Universal Recipient, this means they can receive any blood type in transfusion

Polygenic Traits Some traits are affected by many genes at the same time These are called Polygenic Traits Ex: Height, Skin Color, Eye Color Allows for huge variation in a poplutation Graphs of these traits for a given population result in a Bell Curve

Meiosis Meiosis is the process of making gametes The result of meiosis are 4 sex cell with HALF the normal amount of chromosomes for that organism A normal number of chromosomes = 2N (Diploid) In humans, there are 23 PAIRS of chromosomes or 46 in total Each chromosome in a pair has the SAME genes, one came from mom and the other from dad These chromosomes are HOMOLOGOUS Half the normal amount (found in sex cells) = N (Haploid) In humans, this would be 23 individual chromosomes This allows two gametes (sperm and egg) to combine and form a NEW cell with the complete number of chromosomes

Meiosis Continued During Prophase I of meiosis, Homologous Pairs join together to form Tetrads Remember, one chromosome from each parent with the same genes Chromosomes in each tetrad then exchange small segments of DNA This process is called “Crossing Over” Results in new trait combination Allows for genes to be inherited separately from one another (Law of Independent Assortment)

Process of Meiosis Meiosis is broken up into two stages Meiosis I and Meiosis II During Meiosis one: Prophase I: Same as mitosis, however, chromosomes form “tetrads” Metaphase I: Tetrads line up in the middle of the cell (23 pairs line up in the middle) Anaphase I: Tetrads are ripped apart allowing one chromosome from each homologous pair to move to opposite sides of the cell Telophase I, Cytokinesis I: Same as mitosis

Process of Meiosis Cont. Meiosis II THERE IS NO INTERPHASE BETWEEN MEIOSIS I and MEIOSIS II Prophase II: Same as mitosis prophase Metaphase II: The 23 chromosome “x”s line up in the middle of the cell Anaphase II: The 23 chromosomes are ripped in half allowing 23 chromatids to move to each side of the cell Telophase II and Cytokinesis II: Same as usual, however, at the end, the daughter cells have 23 individual chromosomes mQS_FZ0&feature=PlayList&p=D2A007AC2BF1878B&pla ynext=1&playnext_from=PL&index=23 mQS_FZ0&feature=PlayList&p=D2A007AC2BF1878B&pla ynext=1&playnext_from=PL&index=23