Introduction 1. Mendel brought an experimental and quantitative approach to genetics 2. By the law of segregation, the two alleles for a character are packaged into separate gametes 3. By the law of independent assortment, each pair of alleles segregates into gametes independently 4. Mendelian inheritance reflects rules of probability 5. Mendel discovered the particulate behavior of genes
The current theory for the mechanism for the transmission of genetic material was the “blending” hypothesis. This hypothesis proposes that the genetic material contributed by each parent mixes in a manner analogous to the way blue and yellow paints blend to make green. Introduction • Mendel proposed an alternative model, “particulate” inheritance • This proposes that parents pass on discrete heritable units - genes - that retain their separate identities in offspring. • Genes can be sorted and passed on, generation after generation, in undiluted form.
Blending Hypothesis: 1800s -suggested that traits of parents mix to form intermediate traits in offspring. Parents Offspring Red flower x White flower Pink flower Tall height x Short height Medium height Blue bird x Yellow bird Green birds Fair skin x dark skin Medium skin color If blending always occurred, eventually all extremecharacteristics would disappear from the population.
Modern genetics began in an abbey garden, where a monk names Gregor Mendel documented mechanisms of inheritance. Gregor Mendel: Established genetics as a science in 1860s. Considered the founder of modern genetics. 1857
Mendel grew up on a small farm in what is today the Czech Republic. In 1843- entered monastery. He studied at the University of Vienna from 1851 to 1853. The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance. mendel
Pea plants have several advantages for genetics. Pea plants are available in many varieties with distinct heritable features (characters) with different variants (traits). Short generation time, little space needed, cross or self fertilize
Pollen hybridize X • He took a true breeding purple flower plant and crossed it with a true breeding white flower plant. • He called these the parent generation (P generation) • What do you think the offspring looked like? ALL PURPLE
Allowed to self-pollinate F1 SAY WHAT??????? F2
Something is being passed from parent to offspring. He called these “Factors” Sometimes you can see “it” and sometimes you can’t see “it”. If you can see it- it is dominant.(T) If it’s there and you can’t see it- it’s recessive.(t) Mendel concluded: Purple flower is a dominant trait and white flower is a recessive trait.
1. Alternative versions of genes (different alleles) account for variations in inherited characters. Different alleles vary somewhat in the sequence of nucleotides at the specific locus of a gene Mendel developed a hypothesis to explain these results that consisted of four related ideas. 2. For each character, an organism inherits twoalleles, one from each parent. • 3. If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance. • The other, the recessive allele, has no noticeable effect on the organism’s appearance.
4. The two alleles for each character segregate (separate) during gamete production (meiosis).
Mendel’s quantitative analysis of F2 plants revealed the two fundamental principles of heredity: the law of segregation&the law of independent assortment.
If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers. Instead, the F1 hybrids all have purple flowers, just as purple as the purple-flowered parents. Law of segregation- the two alleles for a character are packaged into separate gametes
The white trait, absent in the F1, reappeared in the F2. Based on a large sample size, Mendel recorded in the F2plants: 705 purple-flowered 224 white-flowered 3:1 ratio(phenotype) Law of segregation- the two alleles for a character are packaged into separate gametes The reappearance of white-flowered plants in the F2 generation indicated that the heritable factor for the white trait was not diluted or “blended” by coexisting with the purple-flower factor in F1 hybrids.
Law of segregation-the two alleles for a character are packaged into separate gametes This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis. packaged into separate gametes
A Punnett squarepredicts the results of a genetic cross between individuals of known genotype. Two heterozygotes - 3:1 ratio of dominant: recessive phenotypes 1:3:1 genotypes USING THE Law of segregation……….. • PRACTICE: • DRAGON Genetics • #1 & 2 • Genetics Practice #8,17
When crossing two pure breeding plants, Mendel found similar 3 to 1 ratios among F2 offspring when he conducted crosses for six other characters, each represented by two different varieties.
Predicting the genotype of an organism with the dominant phenotype: The organism must have at least one dominant allele, but it could be homozygous dominant or heterozygous. what’s the genotype????? A testcross, breeding a homozygous recessive with dominant phenotype, but unknown geneotype, can determine the identity of the unknown allele.
The law of independent assortment- each pair of alleles segregates into gametes independently • Alleles that are not on the same chromosome • Ex. The alleles for height segregate independently from the alleles of a gene for color. • AS long as the genes are on separate chromosomes, the separation during meiosis is RANDOM.
The law of independent assortment- each pair of alleles segregates into gametes independently -During meiosis, the chromosomes line up randomly during metaphase II. -So, depending on how they line, this determines the alleles in each gamete.
Mendel’s experiments that followed the inheritance of flower color or other characters focused on only a single character via monohybrid crosses. He conducted other experiments in which he followed the inheritance of two different characters, a dihybrid cross. The law of independent assortment- each pair of alleles segregates into gametes independently animation
If no independent assortment: The F2 offspring (in dihybrid) would only produce two phenotypes in a 3:1 ratio, just like a monohybrid cross. This was not consistentwith Mendel’s results. animation
Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr). When sperm with four classes of alleles and ova with four classes of alleles combined, there would be 16 equally probable waysin which the alleles can combine in the F2 generation.
These combinations produce four distinct phenotypes in a 9:3:3:1 ratio. This was consistent with Mendel’s results.
Cross two purebred guinea pigs. Brown-long hair x Black-short hair Give phenotype and genotype ratios. Black-BBrown-bLong hair-SShort-s
Mendel’s laws reflect the laws of probability. The probability scale ranged from zero (an event with no chance of occurring) to one (an event that is certain to occur). The probability of tossing heads with a normal coin is ½. The probability of rolling a 3 with a six-sided die is 1/6, and the probability of rolling any other number is 1 - 1/6 = 5/6. Mendelian inheritance reflects rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss. Each toss is an independent event, just like the distribution of alleles into gametes. Like a coin toss, each ovumfrom a heterozygous parent has a ½ chance of carrying the dominant allele and a ½chance of carrying the recessive allele. The same odds apply to the sperm.
Rule of multiplication to determine the chance that two or more independent events will occur together in some specific combination. Compute the probability of each independent event. Multiply the individual probabilities. The probability that two coins tossed at the same time will land heads up is: ½ x ½ = ¼. Similarly, the probability that two heterozygous pea plants (Pp) will produce a white-flowered offspring (pp) depends on an ovum with a white allele mating with a sperm with a white allele. ½ x ½ = ¼.
Rule of multiplicationalso applies to dihybrid crosses. For a heterozygous parent (YyRr) the probability of producing a YR gamete is ½ x ½ = ¼. We can use this to predict the probability of a particular F2 genotype without constructing a 16-part Punnett square. • The probability that an F2 plant will have a YYRRgenotype from heterozygous parents is • 1/16 (¼ chance for a YR ovum and ¼ chance for a YR sperm).
Examples: An organism had three independently assorting traits: AaBbCc. What fraction of its gamete will contain ABC? 1/8 What about an organism with AABcCc? What fraction of its gamete will contain ABC? 1/4
determine the probability of an offspring having two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are PpYyRr and Ppyyrr. There are five possible genotypes that fulfill this condition: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr. Determine probability of each and then add together. Rule of addition: When more than one arrangement of the events producing the specified outcome is possible, the individual probabilities are added.
For ppYyrr: 1/4 × 1/2 × 1/2 = 1/16. For Ppyyrr: 1/2 × 1/2 × 1/2 = 1/8 or 2/16. For PPyyrr: 1/4 × 1/2 × 1/2 = 1/16. For ppyyrr: 1/4 × 1/2 × 1/2 = 1/16. Therefore, the chance that a given offspring will have at least two recessive traits is 1/16 + 2/16 + 1/16 + 1/16 = 6/16.
THE CHROMOSOMAL BASIS OF INHERITANCE 1.Morgan traced a gene to a specific chromosome 2. Linked genes tend to be inherited together because they are located on the same chromosome 3. Independent assortment of chromosomes and crossing over produce genetic recombinants 4. Geneticists use recombination data to map a chromosome’s genetic loci
It was not until 1900 that biology finally caught up with Gregor Mendel. Mendel’s hereditary factors are the genes located on chromosomes. Introduction
Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form.
CHROMOSOMAL THEORY OF INHERITANCE Chromosomes are carriers of traits and each chromosome could carry the genes for MANY traits. Alternate forms or ALLELES of a gene are located on matched pairs of chromosomes. When chromosome pairs separate in meiosis, each chromosome carries its set of alleles to a gamete. Genes of the same chromosome move together; Genes on different chromosomes assort independently.
He was the first to associate a specific gene with a specific chromosome. Worked with Drosophila melanogaster Prolific breeders & have a generation time of two weeks. 3 pairs of autosomes and a pair of sex chromosomes Morgan traced a gene to a specific chromosome • Produces about 100 offspring per egg lay – good statistics! • Easy/inexpensive to raise • Chromosomes are VERY large and easy to see and locate • Sexes are easily distinguished --female is larger --shapes of abdomen identify sexes at a glance
Morgan spent a year looking for variant individuals among the flies he was breeding. He discovered a single male fly with white eyes instead of the usual red. The normal phenotype is the wild type. Alternative traits mutant phenotypes.
When Morgan crossed his white-eyed male with a red-eyed female, all the F1 offspring had red eyes, The red allele appeared dominant to the white allele. Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring. Surprisingly, the white-eyed trait appeared only in males. All the females and half the males had red eyes. Morgan concluded that a fly’s eye color was linked to its sex. Morgan’s Experiments & Findings
Morgan deduced that the gene with the white-eyed mutation is on the X chromosome, a sex-linked gene. • Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes. • Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white-eyed allele.
Sex-linked Genes Sex Chromosomes 1.Sex-linked genes have unique patterns of inheritance 2.The chromosomal basis of sex varies with the organism
In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex. Males are hemizygous for the X chromosome (XY) Sex-linked Genes Karyotype
If a sex-linked trait is due to a recessive allele, a female will have this phenotype only if homozygous. Heterozygous females will be carriers. Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the trait. The chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose. Therefore, males are far more likely to inherit sex-linked recessive disorders than are females. Sex-linked Genes