Download Mendelian Genetics: Basic Principles of Heredity and Sex-Linked Characteristics and more Lecture notes Genetics in PDF only on Docsity!
CHAPTER 3: Basic Principles of Heredity (“Mendelian Genetics”)
3.1: Mendel Discovered the Basic Principles of Heredity ● Gregor Mendel and his sucess in genetics: ○ 1. Had a good experimental model (peas) ■ They produced easily ○ 2. Took an experimental approach and analyzed results mathematically ■ Looked at the ratios ( 3:1 ratio ) ○ 3. Studied easily differentiated characteristics ■ It is either “this” or “that” Important terms: ● Gene : an inherited factor (encoded in the DNA) that helps determine a characteristic ● Allele : One of two or more alternative forms of a gene ● Locus : Specific location on a chromosome occupied by an allele (A or a) ● Genotype : Set of alleles possessed by an individual organism ● Heterozygote : An individual organism possessing two different alleles at a locus (Aa) ● Homozygote : An individual organism possessing two of the same alleles at a locus (AA) ● Phenotype (trait): the appearance or manifestation of a characteristic
Zygosity : look at genotype! (heterozygous or homozygous)
● Two alleles are present in Diploid cells (sister chromatids have the same alleles) ● Prophase 1 cross-over or not ○ Chiasmata: location where they cross over ■ ONLY CROSS OVER IF THE ALLELE IS ABOVE CHIASMATA ■ The same proportion of alleles even with crossing over, just a new chromosome Monohybrid Crosses: ● Conclusion 1 : One character is encoded by two genetic factors (alleles) ○ No polygenetic characteristics in Mendelian genetics ● Conclusion 2 : Two genetic factors (alleles) separate when gametes are formed ● Conclusion 3 : One allele can be dominant while the other is recessive.
● Conclusion 4: two alleles separate with equal probability into the gametes (each allele can be on either side of the metaphase plate). Punnet Square: ● Filial Generation - F1 - POSSIBILITY OF OFFSPRINGS PROPORTION IN F ● Phenotypic Ratio and Genotypic Ratio with offsprings ○ Genotypic ratio: 1Tt:1tt ○ Phenotypic ratio: 1 tall:1 short Mendel Experiment: ● Fertilized pea plants by himself - removed the anther from one plant since they can be male or female ○ Created a TRUE BREEDING LINE (100% of the offspring will be the same, 100% wrinkled, or 100% rounded) ● Cross-purely wrinkled with purely rounded ○ F1 generation (Homozygous recessive + Homozygous dominant) = 100% Heterozygous only round ○ Self-fertilizes F1 Generation = F2 Generation created ○ Classic Mendelian ratios, approximately 3:1 (Phenotypic), 1:2:1 (Genotypic) - knows the alleles separated and recombined ● A recessive phenotype is hidden in F1 (the only time it is hidden) ● ALWAYS 4 GAMETES IN MENDELIAN GENETICS Monohybrid Crosses reveals the Principle of Segregation and Dominance: ● Principle of Segregation (Mendel’s first law): Each diploid organism possesses two alleles for any particular characteristic. These two alleles segregate/separate when gametes are formed; one allele goes into each gamete. ● Concept of Dominance : If two different alleles are present in a genotype, the trait encoded by one of them - the “dominant” allele - is observed in the phenotype. Dihybrid Crosses: ● Examine two traits at a time ● Alleles located on different chromosomes will assort independently ● When we are looking at two different traits… A is just as likely to go with B as it is with b ( EQUALLY LIKELY ) while on the meta plate ○ 4 types of gametes in a dihybrid cross because they assort independently ○ Separate homologous and depending on how they line up… there could be 4 different results (i.e., Ry, ry, Ry, ry) ○ F1 AND F2 - 9:3:3:1 DIHYBRID CROSS PHENOTYPIC RATIO ■ 9 dominant, 3 heterozygous (dominant recessive), 3 heterozygous (recessive dominant), 1 recessive.
CHAPTER 4:
Sex Cell Life Cycle: ● Sexually reproducing organisms have cells that alternate between haploid and diploid states ● Humans have 23 pairs of homologous chromosomes - 23 pairs of autosomal chromosomes - 1 pair of sex chromosomes (XY or XX) ○ XY - not homology - DO NOT CROSS OVER Gamete Size difference: ● The majority of the animal kingdom has two sexual phenotypes ○ Female gamete (egg) - HUGE ■ Saves cytoplasm in the mitotic division from being so large, that it needs the proteins ○ Male gamete (sperm) - (protein cap with a tail), tiny compared to female gamete Sex Determination:
● XX - female - homogametic ● XY - male - heterogametic ● XX-XO - haploid diploidy sex determination ○ Females are diploid, males are haploid ● ZZ-ZW ○ Females are heterogametic, males are homogametic ● Genic sex determination - NOT CHROMOSOMAL ○ There is a gene that determines the sex ● Environmental sex determination - NOT CHROMOSOMAL OR GENETIC ○ The environment fully decides the sex ● X:A ratio (fruit flies)
○ Determined by the ratio of x chromosomes by numbers of sets of autosomes Hermaphroditism: ● Producing both male and female gametes, so both sexes in the same organisms ○ This is how plants “self fertilize” ● Stigma → female reproductive organ ● Anther → male reproductive organ Chromosomal Sex-Determination Systems: Homosapiens ● X and Y chromosomes pair during meiosis even though they are not homologous (the genes located on each are different) ○ They pair at pseudo-autosomal regions (PARs) ○ No F2 generation because that is INCEST! ○ Y is little - X is large ● COHESIN holds the sister chromatids together during mitosis, and SHUGOSHIN holds them together during meiosis. ● Synapses are the close pairing of homologous ● Chiasmata is where the homologous cross-over ● Tetrad - 2 homologous pairs = 4 Sex Determining Region (SRY) gene: ● It produces a “cascade” of other gene products. It is the primary/first “on/off switch” for male sex-characteristic determination and is on the Y chromosome. ○ Butterfly effect ○ If this does not happen, no future events can even occur (you have to open the main door before you can open the next door) The Role of Sex Chromosomes: ● The X chromosome contains genetic information essential for both sexes ○ At least one copy of an X is REQUIRED FOR SURVIVAL ● The total absence of Y results in a female phenotype ● The male-determining gene is located on the Y chromosome. A single Y, even in the presence of several Xs, still produces a male phenotype. ● Turner syndrome - XO: 1/3000 female births ● Klinefelter syndrome - XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births ● Poly-X Females : many X’s; 1/1000 female births Random X Inactivation ● Creates Barr Bodies - dark spots in the cells (Calico cats - only females) ○ Heterozygous - one characteristic gets shut down while the other gets expressed. It activates LATE because the zygote needs to produce those inactivated cells ○ EARLY activation in cats = 50/50 split of fur color (white and orange for example) ■ mosaicism ● Dosage compensation : the amount of protein produced by X-linked genes needs to be stable (i.e., only one X chromosome can be expressed in one cell
● Biologically there is something significantly different that created these proportions (probability due to chance will be very LOW) Meiotic Non-Disjunction in sex-linked Characteristics ● Discovered by Calvin Bridges ● He finds that (creates more data lol) in the F1 generation cross it produces white-eyed females and red-eyed females (this is the experiment done by Morgan) and this introduces Meiotic Nondisjunction (XXY Female) ● X’s do not always separate which creates XX gametes and then the Y is separated from them ○ 10% chance of nondisjunction occurring ○ 2 homologous and 1 gametes to be produced ● Homologous pairs do NOT separate at the metaphase plate ● Could still see white-eyed females if nondisjunction occurs only if XX and no XXY, this could still create XXY - the phenotype ● XX will never create a red-eyed male - you NEED the Y to produce a male fly!
