Sexual reproduction increases genetic diversity in Episode 37.
Sexual reproduction increases genetic diversity in Episode 37. Chromosomes are tightly coiled DNA (1:00) and organized in homologous pairs (1:45). The three primary sources of variation within the population are chromosome segregation, independent assortment, and random fertilization (2:15). Do you remember Mendel’s laws? Some chromosomal errors cause genetic disorders (3:20). We can use pedigrees to find patterns in genetic traits (4:20).
The Question of the Day asks (5:42) “During what phase of meiosis does crossing over occur? ”
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap Chromosomal Inheritance
Lets Zoom out:
Unit 5 - Heredity
Topic - 5.6
Big idea - Systems Interactions
DNA is a nucleic acid of nucleotide monomers, which differ only in their nitrogenous base. It forms an antiparallel double helix, that coils and condenses into chromosomes when preparing for cellular division. This episode will recap how chromosomal inheritance generates genetic variation in sexual reproduction.
Lets Zoom in:
Quick recaP of chromosomes structure. We need to fit 3 billion nucleotides into one cell. Coiling and condensing is essential. DNA begins to wrap around proteins called histones much like beads on a string. This loosest form is referred to as chromatin and is found during most of the cell cycle. DNA then begins to coil further inward, before supercoiling, and forming loops. These loops are arranged in rosette structures, which stack together in a very condensed manner that can be seen under a microscope. We call these condensed strands of DNA chromosomes.
When observing the human karyotype, or mapping of chromosomes, it's important to recognize that they can be organized into 23 homologous pairs. These homologous chromosomes are a similar size and have similar genetic information. The chromosomes you have in each of your cells right now came from your parent’s gametes, with homologous chromosomes pairing back-up during fertilization. The process of meiosis occurs in sex organs, such as testes and ovaries. and separates the 23 homologous pairs into gametes.
The three primary sources of genetic variation within a population are segregation, independent assortment and fertilization. Imagine that there are 20 students in your biology class and the teacher asks you to pair up with someone that is approximately your height and line up in the middle of the room. Some of you are lined up on the left, while others are on the right. Similarly, homologous chromosome pairs are arranged by spindle fibers during metaphase I at random! There are no rules that say all genes from mom must be on the left to go into specific gametes while dads genetic material needs to be on the right. No. Mendel’s law of independent assortment states that alleles of genes located on non-homologous chromosomes will separate during meiosis I without influencing each other. Mendel’s law of segregation states that the two alleles for each gene will separate during meiosis, as diploid cells become haploid. Finally - each egg and sperm are genetically unique - and the random process of fertilization only adds to the reason why you and your siblings are all different.
Certain human genetic disorders can be attributed to the inheritance of a single changed allele or chromosomal error. For example, nondisjunction occurs when homologous chromosomes or sister chromatids don’t properly separate during meiosis. This may result in nonviable gametes, or some that have extra or missing chromosomes. One example of this is with Trisomy 21, or Down Syndrome - where there is a third copy of the 21st chromosome. Sickle cell disease is caused by a mutation in the hemoglobin gene on chromosome 11. Tay-Sachs disease is an autosomal recessive disorder that damages brain cells with an error on chromosome 15. Huntington’s disease is a progressive brain disorder that presents later in an individual's life, caused by an error in chromosome 4. In contrast, X linked color blindness is sex-linked recessive disorder on the 23rd chromosome. Since males are hemizygous for the 23rd chromosome, inheriting only one copy of the X - they will express color blindness is inherited.
The chromosomal basis of inheritance provides an understanding of the pattern of transmission of genes from parent to offspring. Scientists have come up with a handy tool to analyze these patterns with the construction of pedigrees - males are squares, females are circles and connect with lines to show marriage and offspring through generations. The circles and squares are shaded in to show an affected individual. When analyzing pedigrees - you are looking to see if the trait is autosomal or sex linked, dominant or recessive. Start with recessive first, and look for flaws in logic. For example, if two affected parents have some offspring that do not have the trait - then it can’t be recessive. Because passing on alleles from two homozygous recessive parents only has one outcome. If you start to see a pattern of an unaffected mom and a disproportionate amount of affected sons - might be sex linked recessive. If even one parent to child interaction doesn’t work, then you have the wrong inheritance pattern.
To recap….
The process of chromosomal inheritance creates genetic variation through sexual reproduction. Homologous chromosomes and their alleles segregate and assort independently during meiosis - forming haploid gametes. Fertilization of egg and sperm at random - provides even greater genetic variation. Huzzah! Diploid number restored.
Today’s Question of the day is about meiosis/
Question “During what phase of meiosis does crossing over occur?”
Coming up next on the Apsolute RecAP Biology Edition: Listener’s Choice II