Genetics, Mendel and Units of Heredity

Transcription

1 Genetics, Mendel and Units of Heredity ¾ Austrian monk and naturalist. ¾ Conducted research in Brno, Czech Republic from ¾ Curious about how traits were passed from parents to offspring. Gregor Mendel Mendel s Peas ¾ Mendel s pea plant Pisum sativum ¾ Normally, pea plants self-pollinate and self-fertilize ¾ Mendel cross-pollinated and crossfertilized ¾ Mendel s experiments focused on observable traits or phenotype ¾ Mendel began his experiments using truebreeding lines of peas 1

2 Single Trait Inheritance ¾ Seed color ¾ Initial cross parents are the parental generation ¾ Their offspring are the F 1 generation ¾ All offspring resulted in yellow seeds ¾ ¾ ¾ ¾ ¾ ¾ Single Trait Inheritance F 1 seeds allowed to grow and self-pollinate Yellow and green seeds observed in F 2 generation Yellow and green seeds present in 3:1 ratio Green color referred to as recessive Yellow color referred to as dominant Mendel repeated these experiments with other traits, yielding similar results Hereditary Determinants ¾ Mendel s hypothesis of particulate inheritance ¾ Hereditary determinants are now called genes ¾ Each gene carries DNA, a molecule that encodes the instructional information ¾ Each gene has two versions or alleles ¾ Alleles found in an individual are its genotype and have an effect on its phenotype 2

3 The Relationship Between Genotype and Phenotype Genotype: DNA Carries molecular information (blueprint) of who and what we are, however, DNA is NOT the molecule that gives us our appearance. Phenotype: Expression of Proteins DNA encodes the information for making protein and the resulting expression of the proteins makes us who and what we are. Hereditary Determinants ¾ Principle of segregation ¾ Homozygous individuals have two copies of the same allele for one gene ¾ Heterozygous individuals have two different alleles for the same gene From: How to Read a Punnett Square 3

4 Examples of Genetic Inheritance: Incomplete Dominance Genetic condition where neither allele is fully expressed as a heterozygote (F 1 generation). The heterozygote is expressed as an intermediate phenotype. Examples of Genetic Inheritance: Codominance Condition where two alleles have a shared dominance. The I gene yields 4 blood types: A, B, AB, or O 3+(127<3( EORRGW\SH $ % $% 2 *(127<3(, $, $ RU, $ L, %, % RU, % L, $, % LL Examples of Genetic Inheritance: Multiple Alleles and Polygenic Inheritance Multiple alleles: three or more alleles of the same gene existing in a population. ABO alleles Polygeneic inheritance: one trait that is determined by many genes. Height, weight, hair color, skin color, etc. 4

5 On To Molecular Biology Chromosomes Genetic Material Seen when cell is about to divide Contains a particular sequence of genes Humans have 23 matched pairs of chromosomes 46 chromosomes total (2n) 5

6 Karyotype A picture of the chromosomes in a cell that is used to check for abnormalities. A karyotype is created by staining the chromosomes with dye and photographing them through a microscope. Chromosomes occur in homologous pairs. Same size, shape, centromere location. Same banding pattern, same genes. Karyotype arranges homologous pairs of chromosomes from largest to smallest. Some chromosome defects can be detected by karyotyping. The Cell Cycle Two main phases in the cycle Interphase (G 1, S and G 2 ) cell growth and DNA replication Mitotic phase (M) cell division Four stages of the Mitotic phase Prophase Metaphase Anaphase Telophase and cytokinesis 6

7 Gap 1 cell growth Mitosis vs. Meiosis Mitosis Growth and repair Final number of cells=2 No genetic variation Number of chromosomes=46 Meiosis Reproduction Final number of cells=4 GENETIC VARIATION Number of chromosomes=23 7

8 X-Linked Inheritance Certain conditions are called X-linked conditions because they stem from dysfunctional genes on the X chromosome. Men are more likely than women to suffer from these types of conditions because they only have one X chromosome. X-linked disorders include: Hemophilia Duchenne muscular dystrophy Red-green color blindness 8

9 Autosomal Genetic Disorders Autosomes: chromosomes other than X and Y chromosomes. Autosomal recessive disorders: Albinism Sickle-cell anemia Cystic fibrosis Phenylketonuria Tay-Sachs disease Werner syndrome Both parents must have the allele to have a child born with the condition. not Autosomal Genetic Disorders Autosomal dominant disorders: Polydactyly Campodactyly Huntington disease ALS Lou Gehrig disease Single faulty allele causes damage even with a good allele present. 9

10 Tracking Traits with Pedigrees Geneticists can create family tree diagrams or pedigrees, which can be used to determine if a disorder is dominant or recessive, and the probability of future inheritance. Let s Practice! I II III 10

11 Chromosome Abberations Polyploidy Abberations in the number of chromosome sets. Animals and many plants are normally diploid. Polyploidy occurs when one or more sets of chromosomes are added to the genome of a diploid organism. Lethal condition in humans Some polyploid plants may be more robust (ex: wheat). Chromosome Abberations Aneuploidy Abberations in the chromosome number. Aneuploidy is caused by non-disjunction Non-disjunction: The failure of homologous chromosomes or sister chromatids to separate during meiosis. Genetic disorders caused by abnormal chromosome number: Down syndrome (trisomy 21) Turner syndrome Klinefelter syndrome Down Syndrome Most common form of aneuploidy in human births. Most trisomy 21 is the result of nondisjunction during egg formation. Detected using karyotype analysis. Frequency of non-disjunction (and Down syndrome) increases with the age of the mother. 11

12 Turner Syndrome Abnormal number of sex chromosomes. Women and girls with Turner syndrome have only one X chromosome (monosomy). In 75-80% of cases, the single X chromosome comes from the egg, because the sperm that fertilized the egg was missing a sex chromosome. Klinefelter Syndrome Abnormal number of sex chromosomes. Men and boys with Klinefelter syndrome have a Y chromosome and 2 X chromosomes (trisomy). In about half of Klinefelter cases, the extra X chromosome is from the egg, while in the other half of cases, the extra X chromosome is from the sperm. 12